Lord of Souls continues the events of The Infernal City.
Attrebus doesn't feel what has cut his stomach, spilling all his guts. He and Sul have failed in stopping Vuhon and are cast into another realm of Oblivion. For a moment Attrebus is suspended between realms, and then finds himself in a gray void where the air is full of ash. Sul is nearby but drifts off as the ash continues to fall, making it hard to breathe. Feeling defeated, Attrebus awaits the moment when death will finally take him. In his groggy stupor, he begins to see visions. During one where he saw a doll with his appearance being repaired, Attrebus screams, air entering his lungs once more and pain coursing throughout his body. Eventually he realizes he is somewhere which resembles a gray box with no exit or entrance. Sitting up, a naked Attrebus feels his stomach and realizes his mortal wound has been healed.
An Altmer woman appears behind Attrebus; she tells him that he is in Ashpit, the Daedric Prince Malacath's realm of Oblivion, and introduces herself as Silhansa, from Balfiera. She asks Attrebus to tell her his story, and he explains the events of Infernal City; his and Sul's failed attempt to stop Vuhon, as well as being cast into other realms of Oblivion, namely Namira's, which was where Attrebus had sustained his grievous wound. She asks about his plans, and then what Attrebus knows about Malacath. Attrebus recounts a childhood story where his nurse tells him of how Malacath came to be. It is at this point Silhansa reveals her true nature. She is no mere woman, but in fact the Daedric Prince Malacath, who shows his gruesome form to Attrebus, and also reveals that he is holding both Attrebus and Sul in his hands. Malacath recognizes Sul, who has dealt with him, and his realm, before. Malacath reveals that he has healed them and asks Sul what to do with them. Sul asks that he and his companion be released, and Malacath asks why. Sul replies, "To destroy Umbriel." Malacath doubts this as they failed before, but ultimately agrees when Attrebus cuts in stating they failed because they did not have the sword, Umbra, and mentions that Sul still craves vengeance, which is one thing that Malacath delights in. Sul believes the sword to be on Solstheim, so with a warning to not fail, Malacath sends them to Solstheim.
An orc named Mazgar gra Yagash and an Imperial wizard named Brennus are two members of an Imperial scouting team sent to investigate the rumors of Umbriel. They spot the floating city and determine that it's in a bubble of Oblivion, and as it approaches they are attacked by an army of undead people and beasts; when one of their squad members dies, he rises again and fights against his former allies. The scouting group fights, attempting to break through the dead army.
Colin Vineben, an inspector in the Penitus Oculatus, is abducted by two strange men. Soon, he hears the voice of Letine Arese. She explains that the men were working for her, yet she killed them to avoid them becoming suspicious. She shows him a small wolf tattoo, the branding of Emperor Titus Mede and told him that she was placed in Minister Hierem's office ten years before. She then convinces Colin to help her find out what the minister is doing, as Hierem was the one who had ordered the ambush on Attrebus.
Mere-Glim witnesses the death of one of the skraws, which reaffirms his inner resolve to help the skraws have a better life by replacing the vapors. He begins to train a new skraw, Oluth, and while doing so he finds a "worm" which is growing to take the shape of an Argonian, which unsettles him.
Glim and Annaïg meet to discuss their current status (why they cannot leave Umbriel, whether Attrebus is alive, and the skraw rebellion.) Glim asks Annaig to help him make a substitute for the vapors. She reminds him that they might need to die to stop Umbriel, to which Mere-Glim merely nods.
Colin is summoned by Intendant Marall, who wants to know if Colin has found any evidence linking Umbriel to the Thalor. Colin has not, and asks for permission to investigate it in Black Marsh, which is granted. He discovers that the records on Umbriel in Black Marsh have been censored by Hierem's office and reports this to Letine Arese, who tells him that Hierem had taken a secret trip to Black Marsh. Colin wonders why Hierem would want Umbriel to attack the Imperial City.
Chef Toel summons Annaïg to his quarters and tells her that his kitchen and four others have been selected to compete for the honor of serving Lord Umbriel himself. One of the main competition in chef Phmer, who has a distinct advantage because she'd discovered the ninth savor of taste (such as the ones Annaïg knows - sweet, salty, sour, etc - and the ones unique to Umbriel, such as quick and ephemeral). Toel orders Annaïg to also come up with this unknown ninth savor, although he does not have any samples or hints as to what it might be. Annaïg eventually comes to the conclusion that she needs to steal it from Phmer's kitchen. She sent Glim a note asking for help with that.
Mere-Glim sends the skraws to help and also suggests sabotaging the midden drains as part of the plan to make the rest of Umbriel notice the skraws and get an alternative to the vapors. He sits in the Fringe Gyre and Fhena joins him, and he tells her about the Hist and that the trees in the Fringe Gyre seem similar.
Colin breaks into Delia Huerc's former residence and finds her ghost there. She directs him to her hidden journal, and he reads it. The entries describe how Hierem had performed some sort of ritual with the An-Xileel at the Lilmoth city tree: it involved magic and the word "Umbriel". Colin decides to wait for Arese and tell her what he has learned, and intercepts a Dark Brotherhood assassin waiting for her; she saves him from the second one. They end up making love and afterward she tells him that he can get out of this line of work, but he insists that he wants to do something right.
Mazgar's squad arrives in Mountain Watch, a small town near Cheydinhal, and orders them to evacuate; they will bring them to the larger city for better protection. On the way, Mazgar befriends a girl nicknamed "Goblin". As they draw near to the city, the dead army catches up, and they fight their way through and get the villagers to safety. The following morning, Brennus tells Mazgar that it seems whenever they kill one of the dead, a corpse rises in town. From the steeple of the Temple of Arkay they spot Umbriel itself in the distance, heading toward them.
Annaïg and Glim meet and talk about Umbriel's Hist-like trees and Annaïg suggests destroying them, which Mere-Glim doesn't like, and then the topic turns to Annaïg's plan to steal from Phmer's kitchen. Glim tells her that at least twenty others have tried and been killed, and Annaïg says that she'll try that night. The next day she is summoned to Toel's quarters, where he is waiting with Phmer. Phmer accuses her of stealing from her kitchen - which Slyr had evidently warned her that Annaïg had been planning - and which Annaïg denies. Phmer releases a creature that identifies the scent of her kitchen on the dress that Slyr is wearing (borrowed from Annaïg), and a vial containing the ninth savor is in Slyr's pocket. Phmer takes Slyr away, and Toel asks Annaïg how she had done it: she reveals that she had gone far enough into Phmer's kitchen the night before to taint the dress, but that she had reinvented the ninth savor - the lack of all taste - on her own.
Colin reported to Arese with his findings and offered to speak to the Emperor on her behalf, and she told him that her code word with the Emperor was "jasper". Using the code word he arranged a private meeting with the Emperor and told him of his suspicions and gave him Delia's journal. Mede ordered him to find more substantial proof that they could show the council and gave him a key to Hierem's rooms, and told him that they were delivered a letter from the ruler of Umbriel: that they and the other inhabitants of the Imperial City would be allowed to leave in peace if the city itself was left for Umbriel.
Annaïg decides that her best chance to survive is to escape Toel on her own by moving up in Umbriel and that the best opportunity to do so is through the upcoming banquet. When she next meets with Glim, Glim points out that he's been helping her but that she hasn't repaid the favor, and she helps come up with a plan that will make it look like the kitchens are sabotaging each other that the skraws can later claim responsibility for. Glim meets Fhena in the Fringe Gyre and their conversation makes Glim realize that his attempts to help the skraws will not result in a better life for the existing skraws but instead result in Umbriel producing Argonian-like skraws that can breathe underwater while the existing ones die off, so he kills the growing Argonian-type proforms in the sump. Toel decides that he's had enough of his kitchen being sabotaged and brings Annaïg there; he ends up getting in a fight with the skraws and comes face-to-face with Mere-Glim, and Annaïg kills Toel with the invisible blade before he can hurt Glim. In order to try keeping themselves alive, Annaïg suggests that she, Glim, and the two skraws with them make it look like Phmer's kitchen was responsible for Toel's death.
Lord Irrel, the patron of Toel's kitchen, asks the kitchen whether they could still win; Annaïg insists that they can, and Irrel makes it clear that Annaïg is putting her life on the line by doing so. Yeum, who was one of Chef Toel's two underchef, offers to be underchef to Annaïg, and Annaïg agrees. Their dishes are served to the lords, and Lord Rhel, the personal steward of Lord Umbriel, visits Annaïg afterward, telling her that her kitchen has won and that she will serve Umbriel herself in three days to see if she can please him as she did Lord Rhel.
Wert urgently comes to Mere-Glim and tells him that Lord Ix has sent guards to find Glim. Oluth reveals that he and the other Glimmers have destroyed a tree-root feed and marked it to make it clear that the skraws were behind it, which he thought Glim had wanted, and he gives his life for Glim to escape.
Upon returning to Nirn, Attrebus and Sul find themselves in the wilderness of Solstheim. They discuss Solstheim for a bit - how Skyrim gave Solstheim to the refugees of Morrowind after the Red Year, and Sul assumes that most of his people have settled in the south of the island. Annaïg contacts Attrebus through Coo, and he tells her of his and Sul's experiences and what they had learned of Umbriel.
Annaïg begins to prepare a poison for the Hist-like trees, and when she returns to her rooms, she finds several people there who bring her to Lord Umbriel himself via flight. He praises her for the meal she had prepared and wonders how she seemed to know he was from Morrowind, and then orders her to kill Mere-Glim for the trouble he had caused by making some sort of poison that would only affect him. Annaïg agrees and begins making a poison.
Two days after Sul and Attrebus' return to Nirn, they come upon the small village of Sathil near the coast. They stay in the local inn overnight and go up to the castle above the town - owned by Hleryn Sathil - the next day, claiming to be naturalists hoping to stay there while they worked nearby. Sathil's daughter greets them and has rooms and supper arranged for them. Sul decides they'll wait and see what happens over the next few days before outright asking about the sword Umbra.
Colin visits the library of the Penitus Oculatus and speaks to the librarian, Professor Aronil, about a spiritlike creature that had attacked him when in Delia Huerc's former apartment. Aronil tells him it was a type of daedra, and that the knowledge to summon it resides only in a single book currently - a book that, according to a spell upon the library, was last touched by Hierem. Colin brings Letise to a hidden lair under the Ministry where she can hide until Colin has found the proof he needs. He enters Minister Hierem's quarters through the secret door.
Attrebus, having climbed out of bed, realizes that someone else is in the room: Irinja, a serving maid he had seen in the kitchen of the manor. She recognizes him from the pictures of him in his stories and asks if she can help, and he tells her his tale, all about Umbriel and Annaïg and the fact that he's searching for Umbra. She warns him to not mention the sword, and leaves. Sul questions whether telling her was a wise decision.
Mere-Glim awakens in the Fringe Gyre with Fhena ministering to him; she had asked the trees for medicine and they had provided it. She convinces him to stay hidden there with her for days as he recovers, and he eventually decides that it is time to return to the Sump. As he enters the water, he feels a strange tingling, painful sensation, and finds that his limbs are going numb before losing consciousness.
Attrebus and Sul are guided through the Solstheim wilderness by a Nord named Fruth, who tells them about the wildlife found in the region. A sudden avalanche separates Attrebus and Fruth from Sul, who is joined by Lord Sathil and several of his men. Fruth thinks that perhaps the avelanche was caused by a Frost Giant, which is also what Lord Sathil claims, but Sul has his suspicions. Sathil asks Sul who sent him - if it was "him" - a Daedric prince, as Sul understands it. Sathil's able to detect Malacath on Sul, but not whichever prince he was concerned about, so he promises to leave Sul and Attrebus alone. After the avalanche ends, Attrebus warms up in the manor, and he and Sul talk about what had happened with Lord Sathil. Irinja joins Attrebus after Sul leaves and she tells him that Lord Sathil's son, Elhul, is in possession of the sword - and possessed by it, locked away and hidden. The two, influenced by drink, make love.
Annaïg grieves over Mere-Glim's body when Lord Rhel brings Glim to her, and after the lord leaves, she removes a crystal that has formed in Glim's mouth. She looks into her locket, intending to contact Attrebus, realizes that he is in bed with a woman, and becomes upset at him and at herself for feeling that way about him.
Colin uses an item called a "soul-maze" to trap a Daedra that Hierem had left in his room. He searches the room and find some papers containing a diagram and the word "Umbriel", so he copies them as best he can and brings them to Letine Arese. She tells him she knows someone that can translate the runes, and he's concerned about her leaving considering the threat on her life, but she promises to be careful.
Somewhere in a forest, Brennus heals Mazgar's wounds. The residents of Cheydinhal had evacuated the city, and several of Mazgar and Brennus' colleagues were killed in the fight, and the pair themselves had gotten separated from the others. During their travels they're attacked by a group of the dead, and it begins to look a bit hopeless when Brennus' flame clears and the pair discover that they have been joined by a group of Knights of the Thorn. Their leader, Ilver Indarys, offers extra mounts, and they go to rejoin the group from Cheydinhal on the Blue Road.
Irinja leads Attrebus and Sul down to where she says Elhul is being kept. Nirai Sathil follows them and locks the entrance to the prison (a cage door) behind them, and reveals that she had sent Irinja to seduce Attrebus after Irinja had told her what the young prince wanted. She says that all she wants is for this to be over, for the sword to no longer be in her brother's possession. They fight Elhul and Sul finally defeats him with a spell that causes him to dissolve into acid. They wrap the sword up, but Nirai refuses to let them go unless they leave it there. They argue with her about it for a bit and she decides that even if they do claim they'll leave the sword they'll probably trick her, so she keeps them locked up and leaves. They know that they cannot escape the cage, so Attrebus suggests that Sul use magic to bring them to Clavicus Vile, and Sul does so.
Lord Rhel invites Annaïg to a party in his quarters where she can mingle with the other chefs and artists of Umbriel. She isn't too thrilled with the whole thing, though, and hopes she can send Yeum in the future. When she returns to her kitchen, Yeum informs her that they found an intruder with a knife in the kitchen: Fhena. Fhena tells Annaïg that she came to the kitchen to kill Annaïg for killing Mere-Glim. Annaïg reveals that the poison she made caused his body to grow a crystal similar to a soulgem that held his soul, thoughts, and memories, and that she was using it to grow a proform of Glim in her bath which would be fully formed in a few days.
Attrebus and Sul arrive in Vile's realm, and a talking dog who they later learn is named Barbas leads them to the Daedric Prince. Vile explains that Umbriel was once part of its city (and he refuses to use its new name), and that stabbing the being that calls itself Umbriel with the sword Umbra would allow Vile to bring the city back to his realm. Clavicus Vile tries to get them to give up one of their souls in exchange for sending them to the Imperial City, but Attrebus points out that if Umbriel reaches the White-Gold Tower it seems Vile won't be able to get it back, so that he didn't have a choice, he had to take their offer. Clavicus relents and dumps them at his shrine in Cyrodiil. Sul is knocked unconscious in the fall, and Attrebus drags him to a boat at the shores of Lake Rumare and begins rowing for the Waterfront District. Partway across, he contacts Annaïg and she tells him of everything that has happened to her recently. When he finally arrives at the Waterfront, dead creatures pin him down, and a man who recognizes Attrebus intervenes, telling him that he and his companions are going to bring Attrebus to the palace. Attrebus wakes up in Hierem's office, and Hierem informs Attrebus that he will kill him after he learns how exactly Attrebus is involved with Umbriel. Vuhon, apparently, wants both Attrebus and Sul, which interests Hierem. Hierem claims he has a lot to do and leaves Attrebus as his prisoner.
Mazgar's commanding officer, Captain Arges, ordered the refugees and Imperial soldiers to split into two groups; Mazgar is reunited with Goblin when they are divided into the same group. Once they meet up with other Imperial troops, the ranking officer, Commander Prossos, orders Mazgar to watch over General Takar's forces as they take on Umbriel's dead army and to send a messenger if Takar is successful. He is forced to order a retreat in the end, however, and when Prossos tells Mazgar he's going to reinforce the Imperial City, she insists upon coming along. Brennus reminds them both of Mazgar's original orders to protect Brennus and declares that that means that he's coming along too.
Colin meets with Intendant Marall on top of the walls of the Imperial City. The Intendant tells him that Umbriel will be arriving in two days, and that Attrebus was reportedly spotted being abducted in the Waterfront district a couple days before.
Attrebus wakes in his cell and witnesses Hierem traveling through some sort of portal to Umbriel. Afterward Hierem wakes up Sul and proceeds to question the two, using some sort of magic that causes pleasure for telling him an answer he likes and pain for telling him a wrong answer. Sul eventually tells Hierem that Vuhon fears the sword, so that Hierem's continued questioning of Attrebus would not break the young prince's mind. Sul suspects they might still have a chance: if things don't go Hierem's way with Vuhon, he may give them the sword and send them to Umbriel.
Colin returns to the room where Letine Arese had been hiding, and five days after her disappearance she returns, but says she was not able to find anything about Colin's diagrams. He tells her he suspects that Hierem is holding Attrebus captive, and she decides to come along. Colin and Letine release Attrebus and Sul from their cages and they discuss what to do next. Attrebus tells the others that he noticed a rat disappearing when Hierem returned, so he thinks that if they stand on the sigil that Hierem had used to teleport, they'll be transported to Umbriel when he returns. Sul replaces his usual sword with Umbra, and they wait.
Mere-Glim finally awakens in his new body, and Annaïg tells him what she had done. He's furious and declares that he will no longer be one of her experiments, and storms out. Attrebus contacts Annaïg via Coo and tells her of his and Sul's plan to use the sword Umbra against Vuhon.
Attrebus and Sul wait for hours, and eventually his plan succeeds: he and Sul are transported to Umbriel. Sul immediately stabs Vuhon with the blade, although it doesn't kill him, and they begin to fight. They fall onto what Sul and Attrebus call the "glass forest", a webbing made of a glasslike material that Vuhon can control, and continue the fight there. Attrebus is pinned and doesn't quite see what happens, but suddenly everything shatters around them and they fall into water.
Mere-Glim and Fhena sat in the Fringe Gyre as the Imperial City comes into view. Fhena convinces Glim that Annaïg loves him and was only trying to protect him, and he admits that it's the case. When talking about how the Hist shaped him, he suddenly comes to a realization about where he belongs, and runs off to stop Annaïg from enacting her plan.
When Hierem appears back in his quarters, Colin attempts to kill him, but pauses, suddenly remembering the man he had killed to join the Penitus Oculatus. Hierem hits him with a demoralizing spell, leaving Colin defenseless on the floor wrapped up in his own misery. Letine unsuccessfully tries to kill Hierem, and when Hierem tries to kill Letine, Colin finds the strength to try and stop him. His attack distracts Hierem long enough for Letine to send her daedra after Hierem and kill him. Colin helps Letine up and she stabs him in the ribs. She says only that she's owed something after ten years and that she's going to collect on it, steals the Daedric weapon that Hierem had been using, and leaves.
Annaïg poisons the Hist-like trees, and Glim and Fhena find her immediately afterward. He warns her to stop, that the trees only want to go back home and that if Annaïg goes through with poisoning them, Umbriel and everyone on it, including Mere-Glim, will die. Just as she tries to process all this, her locket alerts her that Attrebus is trying to contact her.
Mazgar, meanwhile, is rowing across Lake Rumare with Brennus and five other men. The dead are in the lake, however, and they kill one of the men in her boat. Eventually she and her troops reach the shore and join in the defense of the city. A mortally injured soldier asks her to end his suffering, and she obliges, though she finds it difficult to do so. She shares a brief rest with Brennus before returning to the fight.
Attrebus awakens in one of the skraw caves. Sul saved him instead of going after Vuhon - but since the sword wasn't working, he thinks something else had caused Vuhon's weakness, and figures it's probably Annaïg's poison. Attrebus contacts Annaïg and tells her what happened. Annaïg tells Glim what is going on, and asks him to bring up Sul and Attrebus. He refuses unless she saves the trees. She admits that she has an antidote, and will administer it once Glim does as she asks. He insists she gives it first since there might not be enough time, and even threatens to use force on her, and then begs that she trust him. They share a memory of when they were children and Annaïg hands him the antidote. Mere-Glim finds Attrebus and Sul, and Wert finds him right at that moment as well. Mere-Glim realizes how sick the skraw is and a sudden clarity comes over Glim, and he drinks the antidote himself, feeling a connection to the trees and the rest of Umbriel.
Vuhon finds Annaïg and says that he had to absorb Rhel and three other lords to remain alive, and he attacks her. Glim returns with Attrebus and Sul, and Annaïg reveals to Glim that she's realized that Vuhon's soul is in the ingenium at the bottom of the sump. Sul summons a winged twilight to fight Vuhon, and Sul, Attrebus, and Glim head down to the sump. Attrebus realizes that they're out of time and reminds Glim that he'd claimed to know how to take Umbriel out of Tamriel using the trees. He tries, but cannot overcome them to command them, so they continue their journey downward. Glim pulls them through the water, and as they descend, Sul goes unconscious, so Attrebus takes the sword from him and holds it ready. They land in a small room, and Vuhon follows and attacks, but Attrebus stabs the ingenium, which stops Vuhon. Vile suddenly possesses Attrebus and points out that Sul and Attrebus knew the price all along. Sul argues that it wasn't supposed to be Attrebus, and Vile stabs him with Umbra. Sul pulls himself farther onto the sword and punches Attrebus, and Vile lets the prince go. Sul tells Attrebus that everything is okay, falls onto the ingenium, and vanishes, giving his soul to Clavicus Vile. Vuhon isn't dead yet, and tells Attrebus and Glim that Vile will probably either let Umbriel fall on the Imperial City or try to interfere with Tamriel while he's connected to it, and that their only chance to save it is to use the trees to bring Umbriel out of Tamriel: he'd blocked Glim's previous attempt. Glim knows that he's telling the truth and agrees to it.
Annaïg and Fhena feel a difference in Umbriel when the ingenium is stabbed. They're reunited with Glim and Attrebus not long afterward. Glim tells Annaïg that she'll be able to leave Umbriel now without dissolving into smoke, and that he is staying with Umbriel himself, that he's found his place. She is upset but realizes that she needs to put his desires before hers. Annaïg gives Attrebus a flying potion and asks him to take her hand.
A badly wounded Colin follows Letise up the stairs of the White-Gold Tower. He comments that he's realized that she means to steal its power. She's stunned to find him still alive, and tells him that Hierem wanted to steal Umbriel's power to make him a god, and now she wants the same. He throws the knife she'd used to stab him, and it hits her in the eye, killing her. Out the window the island is nearly close enough to touch the building when suddenly he sees it vanish. Colin slowly loses consciousness and dies, finally at peace.
Before Umbriel's disappearance, Mazgar and Brennus fight against the dead as the creatures try to get over the wall and Umbriel passes over it above. Brennus receives a mortal wound in the fight, and Mazgar speaks to him, telling him that he'd been right earlier, that she does want children after all. She looks around to find that the dead have stopped moving, and when she turns her attention back to Brennus, she finds that he is dead, and weeps at his side until the sun rises.
Attrebus meets with his father after Umbriel's disappearance. Titus Mede tells him that he is a hero, and Attrebus doesn't feel like he is, but Titus points out that he'd saved all of Tamriel despite being afraid the whole time. He said he'd make sure that everyone knew exactly how Attrebus had stopped it, but Attrebus says that he's been given enough credit for things he didn't do, and that the Synod and College of Whispers could take credit. Later, he and Annaïg ride and talk about their feelings for each other, and they decide they want to spend the years together.
Fhena and Mere-Glim sit together, and Mere-Glim comments that he thought they'd be brought to Clavicus Vile's realm, but Fhena points out that while the city was from his realm, the trees weren't, so they'd been returned to wherever the trees were from. Glim knows that all of the skraws and other workers are free and at peace there, and he's eager to explore, but decides to first spend the night in Fhena's arms.