Here is what happened. I wanted to get the nice speed boots Renal Bloodscalp was wearing. I attacked him at his base and this guy called Arkanis Gath appeared out of nowhere and 1-shot all my party members.
AFAIK he could not be killed in BG2:EE so I tried to kill Renal and run the hell out of the place so many times. It was very difficult because Arkanis Gath kept switching targets to my MC or party members unprotected by death ward and he has a short sword with a range longer than bows. He automatically detects invisibility even if you are wearing cloak of concealment. I even used enfeeblement(lvl 5) spell successuly but he kept attacking(!!) despite being enfeebled.
After many tries, I finally managed to escape him and left him on the first floor of the Thieves Guild using one of party members as a sacrifice, because otherwise he would chase me all the way outside the building. Unsuspecting, I saved my game and continued doing quests. (I only use 1 save slot to prevent save scumming)
Then Bodhi told me to kill the shadow master... except Arkanis Gath is standing right there on the first floor near the entrance. Ever since I've been trying to kill Arkanis Gath using all kinds of methods to no avail. His hp never drops below 1, and at hp 1 he is immune to stat/level draining, poison, and all elemental/magic/weapon damage types. Magical status does not affect him, and even when surrounded by summons, he somehow teleports to next area if you wait 1~2 seconds. (I had to use haste + speed boots to run away) Even ctrl+y cheat can't kill him. I'm stuck on my quest.
Is there anyway to kill him in game? If not possible, is it possible to unsummon him or delete him from my save using the keeper? I usually sublate from cheese tactics but as long as this guy is standing at the entrance not even a trio of Mellisan + Demogorgon + Irenicus can infiltrate the thieves guild; more likely they will all get 1-shot to death in one sec. Bodhi is asking for the impossible and my save seems ruined.