Aec'Letec is the final 'boss' following the completion of Durlag's Tower. It is a very hard demon to defeat, mainly due to the special abilities it uses which will cause characters to be dismembered with no save (die permanently, cannot be raised) and turn into hostile ghouls.
As a denizen of the lower planes, Aec'Letec has a very high Magic Resistance, making it difficult to harm it with spells. Spells such as Web or Blindness may still be used to hamper it but whether or not they take effect is a matter of luck. Aec'Letec is also completely immune to fire and cannot be harmed with normal weapons. Enchanted weapons and missiles must be used.
Aec'Letec also has the ability to avoid death by taking over the bodies of its worshipers. If it is killed while any Cult Guards are still alive, it will re-incarnate at full health from the body of a Cult Guard and the fight will continue until no Cult Guards are left alive. The priority therefore is to kill the cult guards before focusing on the demon.
Location
Aec'Letec can be found in the basement of the two-storey warehouse on the western side of Ulgoth's Beard after the Soultaker Dagger has been stolen from the party by cultists.
Strategies
Upon entering the Inner Sanctum, an NPC named Tracea Carol will start talking to you. As soon as this dialogue is finished the fight will start. Aec'Letec will spawn in the center of the red circle, and there are a number of Cult Guard standing passively around the outside.
Party Configuration
You will probably want at least two clerics or two mages, or one cleric and one mage in order to deal with his gaze attack. Key spells to have memorized are Dispel Magic (Level 3 wizard/cleric), Animate Dead (Level 3 cleric) and some form of Remove Fear.
Death Gaze Attack
Aec'Letec will cast a spell on your characters which first acts like a Hold Person spell, and will also add a debuff named 'Dying'. This can be countered in only three ways.
1: Dispel Magic
The easiest way is by use of the 3rd-level cleric and mage spell Dispel Magic. Yeslick is good here for his special innate ability to cast Dispel Magic. In the Enhanced Edition, Inquisitor paladins also can gain innate, rapid and highly effective daily uses of Dispel Magic. Using arrows of dispelling by having one party member shoot another also works. Regardless of delivery, however, it will also dispel any buffs you might have cast.
2: Potion of Mirrored Eyes
Potion of Mirrored Eyes is confirmed to make characters immune to Aec'Letec's Death Gaze. This will prevent said character from turning into a ghast for the duration of the potion. Sorcerous Sundries in Baldur's Gate is one of the places that sells them. Note that the potion does not dispel the Death Gaze attack, so consuming one after a character has been affected by it will do nothing.
3: Mercy Killing
Another, and less preferable, way to prevent the permanent death of a character affected by the gaze is to kill the character before they turn into a ghast. It is not advisable to do this if the protagonist is the recipient of said gaze, for obvious reasons.
Vampiric Touch
Aec'Letec will frequently cast this spell on anything that comes into touch spell range. It heals the demon and increases its survivability considerably when anything other than undead minions attack it.
Distracting Aec'Letec
Summoned Skeletons are very good at keeping his attention (and the occasional gaze attack) away from your valuable party members. If he lands a gaze on any target, it will, after a few seconds, dismember and turn into an undead ghast, preventing any form of resurrection, so it's best to let a summoned creature die instead of them.
By summoning undead creatures directly on top of Aec'Letec, then dispelling magic whenever he casts his gaze on a party member, the characters in the party should be free to move around the outside of the circle killing the Cult Guards. If you run out of skeletons to summon, casting Otiluke's Resilient Sphere on the target he is attacking can keep him occupied for a while. Once all the Cult Guards are dead, it is a simple matter of finishing off Aec'Letec for good.
Aec'Letec is worth 16,000 XP. Each Cult Guard is worth 500 XP.