Underdark Eastern Tunnels Overview - Baldur's Gate 2
The Underdark's eastern tunnels will lead you into a Mind Flayer city. Upon entering this zone you'll promptly be captured by Mind Flayers and the Captured By Mind Flayers side quest will begin. In order to escape this city you'll have to pay a visit to the Master Brain and defeat it, you can find the Master Brain in the northwestern most portion of this map. For more information about how to escape this place check out my Captured By Mind Flayers quest walkthrough.
As for the notable loot in this place, Hilt of the Equalizer can be found in the jail which is used in the Cromwell's Blacksmith side quest. You can also find Methild's Harp and a Ring of Fire Control in this zone, the harp is decent if you have a Bard in your party and the ring is an absolute must have.
Prison: When you are first captured by the Mind Flayers in this city you'll be put into this prison. Aside from your group there is a group of Githyanki that you can talk to in the adjacent cell. Once you get free reign of this area you'll want to grab the Hilt of the Equalizer which is found inside of a container in this room.
Illithid Serum Room: As the name of the room suggests, this is where you'll get the Illithid Serum which you'll need to use on the machines in the Slave Room. In this room you'll also have a Mind Flayer that will respawn once every couple of minute. You may or may not want to use Mind Control on him so you can open the locked doors throughout the compound.
Slave Room: Inside this room you'll find all of the Mind Flayer slaves along with a machine that you can use to make Control Collars. These Control Collars can be used on the Mind Flayers on this map; once a Mind Flayer is under your control you can then use it to unlock the doors throughout this map.
Master Brain: To escape from the Mind Flayer city and back into the Underdark you will have to defeat the Master Brain inside this room. So long as the Master Brain lives the front doors leading back to the Underdark will remain locked.