So, I worked out a strategy to take him down, mostly involving lots of buffs, cloudkill, and fireballs to kill the cultists quickly, but every time I kill him my main character suddenly dies and becomes a ghast. Is this normal behaviour, or a bug? And if it's normal, how can I prevent it?

I'm running BGT, plus the fixpack, with no other mods.

EDIT: Never mind, I just found a Walkthrough that explains the death. Apparently protection from petrification prevents it. Try, try, try again...

Well, to everyone else:

the absolute easiest way I've found is cast invis, walk in, hide in the corner, cast polymorph self into mustard jelly, and then wand of cloudkill. With boots of speed on your little jelly can race around through the cloudkill and Aec'letec will just look at you and die.

I'm sure you found this solution since I found it with a google search, but if you didn't try what this guy did

http://www.ironworksforum.com/forum/showthread.php?t=3408

To be honest I've never bothered to go that far into the TOTSC questline, so I can't say how useful it will be.

hi there today i killed him very easy compared to the last time i fought him the main thing that allways was a problem for me with that demon is that is gaze would freeze my chars , i have two rings of freedom of movement one i got it in the course of game but one other i stole it from a gnome woman in the tree right in front of the tabern , then having minsc or anyone else with the spider bane sword that also gives freedom of movement , give your party the speed spell cant remember in english now xD , but since the rings prevent the spell from working use some speed pots that will work even with the rings , if you have a good tank just keep your tank on the demon wile the other two hopefully or atleast one more with freedom of movement item can kill the other clerics that will turn in the demon if you kill him and there are clerics about , a spell of lv 4-5 cant really recall the name now that basically takes the magic resistance down works great if the demon dont reisist it :P good luck with him like i said this time it was easy but my first time was hell xD

Edit : i forgot to mention that the Gnome woman is in front of the tabern at Ulgoth village :P

I don't think Ring of Free Action have any effect on Aec'Letec's gaze attack. They're not supposed to anyways.

From what I remember, the key to the battle is a lots of scrolls of petrification, and spamming Fireball, Cloudkill, Oil of Fiery Burning, Arrows of Detonation, etc. to take out the cultists. I always have two wizards in my party (imoen and Dynaheir), and I can only imagine that the battle is much more difficult if you only got one wizard. But still, a hasted Coran firing Arrows of Detonation can still do a crazy amount of damage.

Well my friend the ring and the sword Spider bane works wonders againts hes gaze , the first time i did it i only had the sword and it was hard , i had to go down with my mage invisible and clear as much as i could before coming with the rest of the group , but this time i just when down all out on the cultists and theyr demon and had no problems at all , honestly i even was surprised since it took me about 1 hour + the first time i did him and this time was maibe 2 reloads :)

I'm late to this thread, but I'm just recently doing this fight, and thought I'd put what I'd learned here.

Aec'letec has a death gaze, that paralyzes a character and infects them with a monster (I'm not sure but I think it was a ghast). You can tell if a character is infected by looking at the status effects on their portrait on the right side of the screen; the icon for the condition looks like a snake. You can cure the condition using a dispel magic (remove curse does not work). If the condition is not cured after a certain period of time, the character is permanently killed as it turns into a ghast. It can be terribly frustrating to win the fight and be sorting through your inventory afterwards only to have Imoen turn into a ghastt, so be sure to keep an eye out for the condition and be sure everyone who can has dispel magics memorized before this fight.

The ring of free action and spider's band sword are useful because they nullify the paralysis, but they do not nullify the condition, so you still need to cast dispel magic on someone before they die. The spell protection from petrification doesn't protect one from the condition; I'm still experimenting but so far I don't know of anything that does.

P.S. This is using the BG1 engine, with some of the mods from " A Conservative Modder's Guide to Baldur's Gate 1" installed.