Meeting Thisobald Thorm is one of Baldur's Gate 3's stranger encounters, as you regale the monstrous barkeep with your exploits, while knocking back his suspicious-looking brew. The patrons of The Waning Moon, where you find Thisobald in the Shadow-Cursed Lands, have all been transformed, but none so much as Ketheric's son.
While you're exploring The Waning Moon, you can also grab the hidden ledger for the He Who Was quest, plus there's a very fun greatclub called Punch Drunk Bastard that becomes stronger if you're drunk. That said, here's how to beat Thisobald Thorm in Baldur's Gate 3—whether by outdrinking him, or straight up starting a bar fight.
Outdrink Thisobald Thorm
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Your first, and best option, is to outdrink Thisobald Thorm. Speak to him near the bar and he'll challenge you to drink and tell him stories of your exploits. You'll need to drink three times overall, passing Constitution checks of escalating difficulty (14, 16, 18) or a difficulty rating 18 Sleight of Hand check, as you either drink or pretend to drink. If you fail the Constitution check, you'll get Drunk, giving you a disadvantage to Dexterity and Charisma rolls, which makes this next part a lot harder.
See, once you drink, Thisobald wants to hear a tale from your adventures. Depending what you've done in the game so far, you'll be presented with a variety of options with different Performance difficulty ratings. If you fail the Performance roll, you'll have to roll a DR 18 Deception roll to make a story up. Your best bet is to use Shadowheart's Enhance Ability spell to get Advantage or Guidance for a bonus. After the first drink, you can choose:
From aboard a Mind Flayer Vessel, I've witnessed the hells themselves (DR 21)
I gazed on nature's bounties in a verdant grove of druids. (DR 18)
I've seen the goblins' vile rituals with my own eyes (DR 18)
A silver-tongued devil bade me feast in his lavishly appointed banquet hall (DR 18)
I explored a lost fortress buried in the underdark's bowels (DR 16)
The best option is, of course, that difficulty rating 16 choice that comes from finding the Selunite Outpost in the Underdark after solving the Defiled Temple Moon puzzle. There is also a second difficulty rating 16 option that comes from exploring the Arcane Tower. After your second drink, Thisobald will want to hear about tough enemies you've vanquished:
I ripped apart winged imps spawned from the fires of Avernus. (DR 21)
I conquered infamous hobgoblin Dror Ragzlin and his minions. (DR 18)
I slayed a hideous, pustule-ridden hag. She died a gruesome death. (DR 16)
A towering creature rose from molten lava. I felled it with great wit and might. (DR 16)
I bested an inquisitor - a dreaded champion of the Lich Queen, Vlaakith. (DR 16)
The difficulty rating 16 options here come from defeating Auntie Ethel in the Save Mayrina quest, killing the Lava Elemental in the Adamantine Forge, or the Inquisitor in the Githyanki Creche. There are also two further DR 16 options for fighting Bulette in the Underdark or the Phase Spider Matriarch under the Blighted Village.
After one more drink check, Thisobald will reveal that the key to Ketheric's immortality lies in the Thorm mausoleum, before promptly dying. Make sure to loot his body for the key to the backdoor, so you can head out and steal his stuff, including the Punch-Drunk Bastard weapon.
Fight Thisobald Thorm
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If you refuse to tell stories or drink, Thisobald and all the other zombie patrons will become hostile. He might seem a little daunting at first due to his immunity to all Slashing, Bludgeoning, Piercing, and Thunder damage, but there is a trick to defeating him. You need to attack with Fire, Ice, Radiant, Necrotic, or Poison damage. These will cause Thisobald to chug more of his brew, changing his attacks.
After he drinks a few times, he'll receive the Blacked Out condition, which removes all his resistances, and lets you deal extra damage to him. Your best bet is to fight the other patrons while you wait for him to drink too much, before throwing all your damage his way. Just remember to apply those damage types above to get him drinking.