Mind Flayer Area Overview - Baldur's Gate 2
The Mind Flayer Area in the Temple District Sewers can only be accessed if you have the Sewers Key that drops from Tazok in the Windspear Hills Dungeon. Without this key the secret wall you have to open to get here can't be unlocked. For more information about this dungeon you can follow the previous link or check out the Journey to Windspear Hills quest entry I have as well.
As you could have probably guessed by the name of this area - it's filled with Mind Flayer enemies. If you need help with some strategies for fighting these enemies I recommend you check out my How to easily defeat Mind Flayers Youtube Video. Alternatively, check out my Baldur's Gate 2 Beginner's Guide for some helpful information about how to play the different classes in this game and fight harder enemies.
It's worth noting that this entire area is optional. Your reward for clearing this map will be experience, 2 magical items and the friendships you made along the way. The one upside to this is one of the 2 magical items you get here, Hammer of Thunderbolts +3 is used in creating one of the best weapons in the game, Crom Faeyr.
Loot Stash: In the center area of the final room you'll find a container that has Wand of Wonder and Hammer of Thunderbolts +3 inside of it. The Hammer of Thunderbolts item is used in crafting Crom Faeyr, one of the best weapons in the game. For more information check out my Cromwell's Blacksmith guide.