Investigate and Destroy the Cult of the Eyeless

Find and Destroy the Cult of the Unseeing
Find and Destroy the Cult of the Unseeing, also known as Investigate and Destroy the Cult of the Eyeless, is a subquest started by overhearing an Unseeing Eye priest vouching for his deity in the Athkatla Temple District.

Questline
After you overhear the eyeless priest, a priest of Helm will come up to you and ask you to meet him at the Temple of Helm. This will only happen if you are in the Temple District.

Follow the priest to the Helm temple. There he will ask you if you are willing to undertake a mission. Agree. He will tell you that Keldorn Firecam, a potential companion, will be in the sewers to aid you.

Go into the sewers in the Temple District. Not far from the entrance, Keldorn will be fighting a random undead creature. After he defeats it, he will talk to you. Say you are investigating the cult of the eyeless. He can join your party.

Explore the sewers until you come to a secret door. Go through it. You may need some trial and error in getting it open. Go up the stairs. You will end up in an area that looks, no joke, exactly like the third floor of Durlag's Tower in Baldur's Gate: Enhanced Edition. Explore it. If you have explored Durlag's Tower (in BG:EE), after the room that, in Durlag's Tower, holds the ghost, there is Mekrath, just north of that room.

In the room after the room that Riggilo is in in Durlag's Tower, there is a small room with several Yuan-ti, including a Yuan-ti Mage. This mage will cast Chaos on your party, so this may be a difficult battle. A good tactic would be to cast Chaotic Commands on your party, or at least the most important people in your party.

Not sure if this area is necessary to complete the quest, though.

After that, head to the place that will have the pit full of goblins. This should be just southwest of the place you met Keldorn. Defeat the goblins. After that, head northwest. Go down the northwest hallway that is near the secret entrance to the Durlag's Tower look-alike. You will exit the sewers and appear in the cult's stronghold. Go through the hallways until you come to the place with Gaal and the two elite guards. Beware, the main enemies in this area are shadows.

Gaal will talk to you. Ask if there is a way to join the cult without becoming eyeless. He will give you the key to the lower levels. Go to the hallway that you entered the stronghold through. Open the door on the left side of the hallway (which would be near the top of your screen). You will not be able to open this door without the key. Go down the hallway until you reach the end.

There will be a man there. He will talk to you. After that, go down the south hallway (the one near the bottom of your screen). You will reach the lower levels then.

For some reason, in the lower levels, whenever you pass in front of one of the weird tube-like things, two Huge Spiders and one Wandering Horror will appear in front of that tube and attack. There is a treasure stash in the face of the statue.