The encounter with the bartender Thisobald Thorm in Baldur's Gate 3 is probably the most charming one you can have with a optional boss.
While you fight your way through the Shadow-Cursed Lands in Baldur's Gate 3, you'll find the foreboding establishment "The Waning Moon", with its dangerous looking bartender Thisobald Thorm.
This imposing looking creature is Ketheric Thorm's son, the person you're searching for. Thisobald is a charming companion, as you can have a lovely talk without ever laying a finger on him.
If your bloody heart so desires, you can also fight him. But remember: He is definitely no pushover and a formidable foe. Choose wisely.
Baldur's Gate 3: Drink To Your Heart's Content... Or Rather Fight?
As you take a seat, he will pour you something from his questionable brew. Pick it up and decide if you want to pass the Constitution check and drink it, drink and get drunk, or fool Thisobald and don't drink at all – be ready to reload if necessary, the DCs are quite high.
But, if you manage to beat the DCs and involve yourself in a conversation with him, he demands several stories from you. Anything works, and you always need to pass a Performance check. Continue so long until Thisobald gives you a hint about the Thorm Mausoleum, which you'll need to face eventually.
Once he gives you the hint, he reaches his limits and fades away. If you fail any of the Perception checks multiple times, you need to face him head on. But choosing to drink is a great way to skip the fight altogether, while getting the most out of it.
And let's be serious, having a chat once in a while, and having a memorable time with an optional boss is all worth it. I'll miss you Thisobald, rest in peace.
Fighting Thisobald
If you're so inclined to fight him, make sure you're prepared beforehand, as you'll face a gimmick fight, just like opposing Gerringothe Thorm in the Tollhouse. In order to hurt him, you need to wait for the right moment to strike, while dealing with the surrounding imps and his continuous attacks.
Thisobald starts the fight with immunity to all piercing, slashing and bludgeoning damage, dealing magic damage is your only chance. While dealing any magic damage, Thisobald will add this to his brew, making him immune to even more attacks. However, if he adds multiple elements into it, he'll create a necrotic brew, and knock himself out.
With every passing turn, he will take another swig of his brew and become more intoxicated before he'll "Black Out" at round three or four. With a dramatic scene, like he's playing the dying swan, he becomes vulnerable to any damage type and gives you an advantage against him.
This is your time to hit him with absolutely everything you have at your disposal, as it only lasts one turn. Ideally, you kill him before he wakes up, otherwise the next round could hurt you a lot. And if this happens, keep your party spread out, no matter what, as his AOE attacks deal around 30-40 damage.
Once Thisobald is dead, you can loot him and get a key for his locked room. You'll find tons of alchemical ingredients, some diary pages showing which bond he shared with his father and the recipe for Purple Worm Poison.
Behind the bar is also another little secret hidden. If you have a high Perception, you'll notice a loose plank and find a key item for the quest line "He Who Was". Don't forget it and leave this cozy little hideout.