This achievement is obtained by defeating a spirit of malog bal in the center of the imperial sewers. The area you want to reach is the circular section in the center of the imperial sewers map. Once you reach that area there will be 3 gates you need to close to make him appear. It is highly recommended you do this in a group. Closing the gates can be pretty tough and you might run into other alliances. I made it to the center solo and saved my game. I then came back later in the evening and the center area was full of reds and we easily closed gates and defeated Molag bal for achievement. Hope this helps.