Find the Worshippers of Ghaunadaur - Baldur's Gate 2 Side Quest
This side quest takes place during The Drow Summoning Ritual quest when the city of Ust Natha is locked down. When you go to visit Phaere at the start of the quest you'll be stopped by Taso Kala whose standing outside the Female Fighter's Society. She'll pretty much demand that you go to the platform to the southeast where you'll have to fight a large group of enemies.
You can find the platform just south of Deirex's Tower, we visited this area once before when we had to kill the Beholder with Phaere and Solaufein earlier during the Inside The Drow City quest. I strongly recommend you spend some time buffing up before running straight to the platform as there will be quite a few enemies here waiting for you.
Below you'll find a picture of what enemies to expect. I recommend you focus on the spellcasters first (as per usual) to make the fight easier.
Once you've take out the dozen or so enemies waiting for you return to Taso Kala and speak with her to complete this quest. There is no reward for completing this quest aside from the experience and friendships we made along the way. Now head back inside the Female Fighter's Society and speak with Phaere to continue with The Drow Summoning Ritual quest.