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Defeat Ketheric Thorm is a quest in Act Two of Baldur's Gate 3. It can be started by completing Infiltrate Moonrise Towers.
 | Spoiler warning: The following content contains unhidden spoilers. Read at your own risk. |
Objectives
Objectives and journal entries may vary pending player decisions and outcomes.
- We battled Ketheric Thorm. Before we could finish him, he fled somewhere below the tower. We need to follow him.
- Ketheric learned that the weapon he's searching for and the artefact we carry are one and the same. We've been teleported to a laboratory where Balthazar plans to remove us from the artefact's protection. We need to escape.
- We pursued Ketheric beneath the tower and found ourselves in what appears to be a mind flayer colony. We need to find him.
- We escaped Balthazar's laboratory and found ourselves in a mind flayer colony. We need to search for Ketheric.
- We found Ketheric, along with a Bhaalist named Orin and a Banite called Gortash. They brandished a triad of magical stones that seem to give them control over an illithid elder brain. Orin and Gortash have taken the brain to Baldur's Gate aboard a nautiloid - leaving Ketheric to face us.
- Close to death, Ketheric transformed into an apostle of the dark deity Myrkul - and was defeated all the same. We should search his body for the stone he used to control the elder brain.
- The gem we found on Ketheric's body is a Netherstone - one of three used to control the elder brain. Orin and Gortash carry the other two. If we want to cure ourselves and save the Sword Coast, we'll have to hunt the pair through Baldur's Gate and claim each of their Netherstones in turn.
Walkthrough
Depending on the player's actions in Find Ketheric Thorm's Relic and Resolve the Abduction/Capture Isobel the Selûnite Cleric, the party's state after going on the rooftop of Moonrise Towers is very different.
If the party worked with Balthazar and sent the Nightsong to Ketheric Thorm and Isobel was kidnapped or killed, then the party stands judgment before the Absolute and is taken to Balthazar's Necrotic Library after Ketheric realizes the party possesses the Astral Prism. When the party comes to, they are trapped in Profane Wombs and must escape before Balthazar can complete the ritual. After escaping, Balthazar must be defeated. He is joined by several undead servants. If the party killed Dror Ragzlin and Minthara, then their undead corpses are among them.
If the party saved or killed the Nightsong, then Ketheric fights the party on the rooftop and, before his hitpoints are fully depleted, a tentacle bursts out from the tower and causes him to disappear. The party must follow him down the Hollow Tower and into the Mind Flayer Colony.
The main objective of this quest is to the east of the Hollow Tower entrance, or northeast of the Necrotic Laboratory. Use the necrotic apparatus to descend to the heart of the colony.
Optional Activities
The following activities are not strictly required to complete the quest, but can offer additional insight, loot, and permanent buffs.
Recruiting Us
If Us was recruited on and survived the Nautiloid, they can be found in the Morgue, caged by a creature named Chop. The party can stealthily lockpick the cage's door (DC 14) or attempt to persuade Chop to free Us. The party can use an Illithid Wisdom check (DC 14), a Persuasion check (DC 14), or an Intimidation check (DC 18). Failing the check makes Chop hostile.
Slack-Skinned Head
Off to the side of the Necrotic Laboratory is a Slack-Skinned Head. Combining a brain jar with the machine next to the head allows the mind to speak. Most of these heads are simply stories of victims of Gortash, but the Waking Mind is the mind of a Githzerai that knows it is trapped. It offers to provide a Githzerai Mind Barrier to the party if they agree to free it. If the party agrees, the character that frees the mind gets a permanent 
buff.
The locations of the minds are as follows:
- Waking Mind (X:749, Y:-138) - In the Necrotic Laboratory behind the door locked by the mind puzzle
- Butchered Mind (X:678, Y:-68) - In the Morgue on a shelf
- Closed Mind (X:716, Y:34) - In the Barracks in Ironfingers' room
- True Mind (X:696, Y:-116) - Right in front of the machine
- Willing Mind (X:693, Y:-115) - On the shelf next to the machine
- Fresh Mind (X:750, Y:-8) - On a box near the elevator to the Thorm fight
Additionally, the Dark Mind and Slave Mind from the Prologue can be used here.
Mind Puzzle
Th Mind Puzzle is on the southern end of the Necrotic Laboratory (X:737, Y:-112). Interact with the console to learn it yearns to connect, choose to connect with it to start the puzzle. The puzzle involves connecting the various senses on one side to the corresponding sense on the other side of the nodes. Completing the puzzle opens up a locked door which contains the Waking Mind and a Blade of Oppressed Souls. There is also a Desecrated Relief that sheds more light on the Grand Design.
Freeing Prisoners
Within the Tadpoling Centre, a number of not yet infected prisoners can be set free, at the risk of also releasing a number of Mind Flayers. This is part of Find Zevlor. North of the prisoners is Zariel's asset, who can be freed as part of Wyll's companion quest, The Blade of Frontiers.
Kressa Bonedaughter
Kressa Bonedaughter is in the Barracks, giving a speech to Hairy Henry, Ironfinger, and Maghtew Budj. If the party gets too close to her, she calls them out as most likely being intruders. She can be talked down with a Deception check (DC 21) or simply fought. For Dark Urge characters, there is unique dialogue with Kressa that reveals more about their past life, but a fight is unavoidable with her specifically for this origin.
Locating the Chosen Three
Once the party has located the Neural Apparatus on the eastern end of the Colony, they must descend to face Thorm. After descending and opening the door, a scene plays which reveals that Thorm is joined by two collaborators - Gortash and Orin the Red - as well as a captive Grand Duke, Ulder Ravengard. The trio use Netherstones to control an Elder Brain using a Netherese crown. If Gale is in the party, he must be talked down from sacrificing himself by detonating his Netherese Orb, otherwise the game ends early with an ominous note that mass-ceremorphosis has occurred all over the Sword Coast.
Approach Thorm, who threatens the party. There is one last chance to speak to Thorm. Using a Persuasion check (DC 18), he can be talked down somewhat - skipping the first half of his fight. Otherwise, Ketheric fights the party to the death. If the Nightsong was captured or freed, they need to go to her and use the help action to free her. Additionally, if Isobel was kidnapped, she is there alongside Thorm and fights with him. After the first half of the fight, Thorm falls into a pit and transforms into the Apostle of Myrkul. Defeat it and Myrkul abandons Thorm, who finally dies for good. Take his Netherstone and go to the portal to return to the ground floor of Moonrise Towers. Thorm has been defeated and the party can move on to Baldur's Gate for Act Three.
Quest Rewards
Achievements
No Free Lunches
Defeat the Apostle of Myrkul before it consumes any necromites.