In Baldur's Gate 3, General Ketheric Thorm is introduced to players formally around the middle of Act 2, once traversal of the deeper Shadow Cursed lands becomes available. The invulnerable General of the Absolute isn't so tough once players rescue the Nightsong from the Shadowfell, cutting off his source of immortal powers.
After getting through Moonrise Towers' many True Souls and other enemies, players will find themselves face-to-face against a weakened Ketheric Thorm. But don't take this BG3 boss fight for granted – though the first phase is relatively easy, the second phase is one of the game's most difficult, yet cinematic, boss battles.
Phase One: Tips To Beat Ketheric Thorm
Use Turn Undead & AoE CC To Negate Necrolytes
During the first phase of the fight with Ketheric Thorm, the General will regularly summon Necrolytes to bolster the enemy roster. The Necrolytes aren't too much of a threat – not enough to draw away focus from Ketheric himself – but some AoE abilities that target Undead can be incredibly useful here.
The number one choice for any party should be Turn Undead, a Channel Divinity power available to Clerics. A Life Cleric can use Turn Undead in the middle of the battlefield atop Moonrise Towers to practically take away the Necrolytes from the fight, literally turning them away and dealing a huge amount of damage to each one.
Thorm will sometimes use Incubate Death to create new Necrolytes after a turn of growth. It can be worth taking out these growths to turn the Action Economy towards the player's party's favor, as the enemy team will have overall fewer turns for actions. In general, though, turn to AoE for the adds in the Ketheric Fight, and consider using spells that do extra damage or effects to Undead.
Keep The Nightsong Healthy & Buffed
The most important part of this fight is to keep the Nightsong alive. Dame Aylin is responsible for the vast majority of damage output in fights she takes part in, but despite her being the daughter of a Goddess, she can still be taken out of the battle if her immense HP drops to 0.
Keep a party member on support for the Ketheric Thorm boss fight, and make that party member focus on buffing and supporting Nightsong above other party members. You will find her to be a powerhouse of damage, able to nearly single-handedly take down Ketheric in a matter of turns with the right setup. However, Ketheric can't actually be killed in this first phase.
Focus Ketheric – The Tough Part Ends When He Reaches 40% HP
General Ketheric Thorm, now robbed of his invulnerability with the release of the Nightsong from the Shadowfell, is mortal. He can be hurt, and killed, but not finished in this first phase. Instead, once players bring him down to 40% HP (which changes depending on your difficulty mode), the fight will end with a cutscene.
This concludes the first phase of the Ketheric Thorm boss fight – after speaking with Jaheira and cleaning up the mess, jump into the bowels of Moonrise Towers to find the Mind Flayer Colony. Here, players can explore to finish a number of questlines, and learn the truth of the Absolute (if they haven't already) when they meet the three that control it.
This pivotal moment ends when Ketheric sacrifices himself to become the Apostle of Myrkul, an immensely powerful enemy, one shrouded by the God of Death, and the true final boss of Act 2.