One of the big cast announcements of Baldur's Gate 3 prior to the release of the game was that J.K Simmons would be voicing Ketheric Thorm, one of the main villains of the game, and the main villain of Act 2. He's a daunting task to even the hardiest of parties, with immortality being granted to him through the use of something known as the Nightsong, but if you can take that away then he becomes a mortal man. Then, the real battle begins.
Getting To Ketheric Thorm
Before you get to the fight against Ketheric Thorm at Moonrise Towers, we'd recommend that you go to the Gauntlet of Shar and free the Nightsong which is underneath the Temple. Once you've done that, Thorm loses his immortality and becomes a man that can be harmed by normal means, and will also have to face down the Nightsong herself.
Once you get through the hordes of enemies waiting for you and the Harpers as you enter Moonrise Towers, you'll have to face up against the big guy himself. Try and explore the Towers first though, as it'll give you a nice advantage once you get to him.
Once you get up there, you'll see the Nightsong absolutely laying into Thorm, assuming you freed her and didn't kill her. She's not happy with what he did to her at all and is ready to kill him. You're given the option here to talk to Thorm, and assuming you can pass a check that the game gives you you can try and talk him into repenting for his sins. It won't work though, and you'll be thrust into the first phase of the fight against Thorm and his minions. Don't worry though, doing this has an effect later on.
Ketheric Thorm First Phase
The first thing you should do here is take out the necromancer that is assisting Thorm. She's an extreme annoyance if you keep her around, but you can easily tank hits from the risen skeletons that are dotted around the tower. Once she's dead, concentrate ALL of your firepower on Thorm. It's not worth beating on the Skeletons, but getting Thorm down to a certain health threshold will trigger a cutscene that will lead to a massive mind-flayer ship coming through the tower and ending that phase of the fight. You can heal up now and jump into the hole after Thorm, but be wary that once you enter the ship it's not like you can just leave.
Entering The Mind Flater Ship
Now that you're on the ship you can explore and you'll find some interesting things, along with some side quests culminating here, but make your way through the ship until you actually reach an entrance to a chamber. The game will warn you that beyond this door lies something dangerous, which you can probably infer to be Thorm himself. Go through the door.
Completely Skipping Phase Two
This is where things get really interesting. There's a big cutscene here that explains a lot of the lore around what's happened so far and what's going on exactly with Thorm, but once it's over you're faced with Thorm himself. If you did exactly as we stated you should and tried to persuade him, you'll be able to guilt him into allowing himself to die, completely skipping a second phase of the fight that you would otherwise have to fight. Unfortunately, this releases an even more terrifying beast. You see, Thorm is the avatar of Myrkul, the God of Death, and he re-emerges as the Apostle of Myrkul, a massive Skeleton with incredible abilities. This will start the third and final phase of the boss fight.
Ketheric Thorm Boss Fight Phase Three
Here, you need to free the Nightsong as quickly as possible. Do anything it takes to get to her and free her before you concentrate all of your firepowers on the Apostle of Myrkul himself. With the help of the Nightsong, you should be able to deal a whole host of damage to him but remain aware that the Bone Chill aura is active which means you can't heal at all, and if you let the Apostle consume one of his Necrolite's he'll be able to use the Finger of Death spell, an ability that can do a LOT of damage and basically instantly kill the target. Still, if you can aim all of your firepower at him then you should be able to take him down rather quickly.
If you're having issues with the fight, try using as much radiant damage as possible. Bringing Shadowheart down here can come in handy, as between her and the Nightsong you'll do a TON of extra damage to him, and quickly take him out for the final time.