Heretic Temple Overview - Baldur's Gate 2
The Heretic Temple map is part of Rasaad's Companion Quest and is unlocked by either killing or interrogating Hammerhelm on the Abandoned Amphitheater map. In the outside portion of this map you will find a couple of random events that you can participate in but only Wilson is of any importance.
Skylight: You can climb down this Skylight and into the inside of the Heretic Temple using some Rope from the Bound Monks nearby. This will reward your entire group with 2k EXP.
Wilson: Here you will find Wilson, a bear who is involved in the Where's Wilson? side quest. Once you complete Rasaad's Companion Quest you will lose access to Wilson and no longer be able to free or recruit him so it's extremely important you do it now if you want it done at all.
Bound Monks: At this location you will find a group of 3 Bound Monks whom you can convince to abandon joining the cult.
Lemzenn: Lemzenn is a Dark Moon Monk spy who you can talk to. The best dialogue options to choose for him are 3, 4, 3, 2, 1, 1 - this will earn you 1k EXP for each member of your party. Choosing most other dialogue options will turn Lemzenn hostile and result in combat.
In the inside portion of the Heretic Temple you'll find 4 different tests each of which you can participate in during Rasaad's Companion Quest. There are a few more rooms inside of this temple as well that contain some items inside of chests but it's mostly all vendor trash.
Collus: When you first arrive here you'll have to speak with Collus for a scene and to advance the story forward. After you've finished all 4 of the tests in this area you'll have to talk with the Master of Combat who spawns in the middle of this room too.
Room of the Bright Moon: To properly pass this test you're supposed to follow the path of light until you get to the center of the room. Once you reach the center you'll be asked by "The Truth" to tell him a secret, all of the available options will work just fine. It's worth noting that if you stand in the shadows you'll spawn Shadow enemies that will attack you. You can do this indefinitely farming as much EXP as you want.
Room of Penance: In this room you will be asked to supply information about someone who is disloyal to the cult. You'll have a few different options depending on what scenes you witnessed outside of the temple, any of the available secrets will work.
Room of Pain: All you need to do for this trial is refuse to submit to the pain and do not provoke the people attacking you. It's impossible to die from the damage that they deal to you.
Room of the Blinding Sun: You'll have to fight and defeat 3 Invisible Stalker enemies to pass this test.