Undercity is a ruined city underneath Baldur's Gate which is infested with Skeletons and zombies. Part of the undercity is the Temple of Bhaal which is where Sarevok and his most loyal followers are. You can get to the undercity in Chapter 7, by going downstairs in the Thieves' Guild and correctly finding yourway through the Thieves' Maze.
Tamoko, can be found near the entrace to the Temple of Bhaal. She doesn't want Gorion's Ward to kill Sarevok and she makes a futile attempt to convince you. You can convince her to leave Sarevok behind or otherwise she will fight to death. She will drop the Plate of the Dark if she is killed.
East of the Temple of Bhaal there is ruin inhabitated by four skeleton warriors, which may be very dangerous. Two of them will drop Two Handed Sword +1.
Rahvin, who leads a party of six assassins (including him), which consists of one ogre, one druid, one mage, and two fighters and one thief who are hired by the Iron Throne to kill Sarevok. They will also try to kill your party, no matter what you say. This is probably your last assassin party you come across in the game of Baldur's Gate and they prove to be the strongest party in the game.