Mekrath's Hideout Overview - Baldur's Gate 2
Mekrath's Hideout aka Mekrath's Laboratory is a map that you'll find in the sewers beneath the Temple District of Athkatla. Inside this dungeon you'll find a new potential companion by the name of Haer'Dalis. You'll be sent to this dungeon as part of the Free Haer'Dalis from Imprisonment by Mekrath side quest to get this companion but that quest is not a requirement to come here nor to free Haer'Dalis.
Aside from Haer'Dalis the only other things worth mentioning in this zone are the Portal Gem which is used for the Free Haer'Dalis from Imprisonment by Mekrath side quest and Mekrath. You can find Mekrath in the western portion of the zone and he'll talk to you at first and so long as you're not hostile with him he won't attack. Talk to him a second time and he'll even offer you an easy fetch quest if you ask him for an adventure.
Portal Gem: During the Free Haer'Dalis from Imprisonment by Mekrath side quest you will be tasked with coming to this zone to free Haer'Dalis and collect the Portal Gem from this altar. I strongly recommend you complete this side quest as it'll unlock the Planar Prison map which has some BiS loot in it.
Haer'Dalis: Haer'Dalis is a potential companion in BG2 and he'll become available once you kill Mekrath and/or complete the Free Haer'Dalis from Imprisonment by Mekrath side quest. Until then he will be under a mind control spell and stuck in this room.
Mekrath: Mekrath is the head honcho of this place. He'll offer you a quest if you ask him about an adventure the second time you talk with him. This is a short side quest that requires you to go into the Temple District Sewers and obtain a Mirror from an Imp. You can get him to leave this hideout and give you Haer'Dalis by finishing this quest or by attacking Mekrath and killing him. Alternatively, you can always do both for the most EXP too!