Before you enter in the Temple of Shar in Baldur's Gate 3, you'll be stopped by the devil Raphael, who asks you to do something for him. He wants you to defeat a demon down in the Temple who has been hiding from him for a while, and if you do so then you'll be able to discover more about Astarion and what exactly his whole deal is. The issue is that the fight is really difficult at the level you'll likely be when you first enter the Temple, but there is a pretty cool way around ever having to pull your weapon out.
How To Defeat Yugnir Without Fighting Him
When you first enter into Yugnir's chambers, you'll be thrust into a conversation with him. Assuming you've put points into Charisma, you'll be able to decipher the terms of Yugnir's deal and persuade him that the terms he accepted mean that he needs to kill his own followers, which requires passing a DC19 Persuasion check. If you're successful then he'll turn his followers into ash, and then you're able to try to persuade him to kill his Displacer Beast.
This requires a higher Persuasion check, with a DC of 21, which means it might be worth using all of the spells you've got that can potentially give your ability checks a bonus. If you pass, he'll kill the Beast, and then you can perform the same Persuasion check with the same DC to try to get Yugnir to kill himself. If you manage to pass it, then you can loot his corpse and everything else without ever having to actually lay a hand on him.
If you fail these checks, then fret not. The fight becomes a lot easier if you pass even one of them, as it means you won't have to deal with a whole section of Yugnir's forces, and if you're really lucky then you'll only have to deal with him. Once you've defeated him, finish up everything that you're doing in the Temple of Shar and report back to Raphael so that he can hold up his part of the deal.