Cult Hideout Overview - Baldur's Gate 2
The Cult Hideout Map is reached by traveling through an exit on the northwestern side of the Temple District Sewers. You can only access this zone if you've accepted the Investigate and Destroy the Cult of the Eyeless side quest which begins in the Temple District of Athkatla. I strongly recommend you read my quest walkthrough before proceeding in this area as there are a few ways you can mess up this quest and permanently close off 3 zones, the Lower Reaches, Ghoul Town and Pit of the Faithless.
In the northern portion of this zone you'll find a powerful Lich enemy that will give you good EXP when defeated as well as an item used in the Restore Kangaxx's Body side quest. You may have to come back to this location later in the game during Chapter 6 if you can't kill the Lich now.
Gaal: The first time you meet Gaal outside of the Cult Compound you will want to talk to him about finding a rod and accept his quest. If you refuse and choose to attack him you'll have to fight all of the cultists in the compound as well as a Beholder who will rise up from the bottom area into the middle of the compound. You shouldn't go this route because it will cause you to miss out on a couple of maps (Ghoul Town, Pit of the Faithless) and a lot of EXP and loot.
Gaal 2: Once you've spoken to Gaal and accepted the quest to find the rod he'll retreat to this room. After you kill the cult's Beholder 'god' Gaal will return to the previous spot with a few guards that will ambush you the next time you venture by. Him and his guards will drop Girdle of Fortitude, The Guide +2 and Bracers of Defense AC 7.
Sassar: After you speak to Gaal and accept the quest you'll get Gaal's Key, this will let you open the door that leads to Sassar. He will stop you as you walk by and update your quest; essentially he tells you to retrieve one half of the rod in the area below him and then he'll tell you how to retrieve the other half. You'll have to return to Sassar later on to learn the secret code which you'll have to tell to Tad in the Cult Compound. This is how you'll gain access to the Ghoul Town and Pit of the Faithless Maps.
Tad: Tad is another one of the important NPCs involved in the Investigate and Destroy the Cult of the Eyeless side quest. Once you learn the phrase 'The Eye is Blind' from Sassar you'll have to visit Tad in the Cult Compound and recite the phrase to him. Doing this will grant you access to Ghoul Town and the Pit of the Faithless zones.
Lich: Clicking on this sarcophagus will summon a Shade Lich enemy. This is a very powerful enemy that you likely will not be able to defeat on your first run through this area. I'd recommend you return to this location after beating the Underdark portion of the game, by that time you'll likely be high enough level. For tips and strategies on how to fight hard enemies such as this lich check out my BG2 Beginner's Guide. Once you defeat this Lich be sure to loot the sarcophagus as you'll find Golden Arm and Leg which are part of the Restore Kangaxx's Body Side Quest.