If your main avatar is Fane (or maybe any undead?) then after entering the Academy on the Nameless Isle you're given a chance to submit yourself to the God King. If you accept you'll gain the "Sworn" tag and your Bless skill is replaced by Curse. Problem is the God King is kindda demanding. And destructive. First of all if you do the Beast quest (or bump into the Secret Court in the Arx sewers) he'll instruct you to release the Deathfog and kill everyone in the city. If you disobey he'll curse you permanently.
Permanent curse is not a huge deal (just some mild debuffs) but in the final scene, before the fight with Lucian and his squad he'll order you to fulfil your Covenant, kill everyone and open a portal to release all the Voidwoken and the God King into the world. Now you have two choices:
1. Accept - what follows is a clusterfuck. Your companions will turn on you and join Lucian so now you're up against like 8 powerful enemies. Alone. No chance, mang. So before the fight I've sacked all my companions and tried to solo Lucian but I can't survive the first two rounds. For one, my Fane is a wizard so he's squishy but also the usual cheat-death tricks don't work which is weird. "Living on the Edge" doesn't work, Lucian just smashes you and there's the "Load last save" screen. Comeback Kid or the Idol of Rebirth don't work either. I suppose that's normal when you're playing solo? Or maybe it's just the final fight gimmick?
2. Decline - the God King ends you. Not even kills, he just removes you from existence, with no chance of resurrection. Now if you enter the fight with your companions and decline to obey the Covenant, the God King evaporates Fane and you have another two choices: a) fight Lucian with the rest of your team, or b) sacrifice yourself.
Problem is if you chose the sacrifice option nothing actually happens. Lucian and his squad are non-interactable, you can run around the room but that's it. Yeah.
So it seems my only option is go into the final fight with all my people, let Fane be ended and try to fight with just three members. Fight seems to be quite long tho, Braccus Rex summons the Kraken and it's just a shitstorm in general.