This achievement is gained by killing the overseer. The Overseer will be the next boss you fight after you defeat Brink. The Overseer is an undead boss who has a rather large group of zombie and skeleton knights protecting him. He is a pretty easy target to kill and it's story line based so you will not miss him. I'm not for sure but I think I found him on the 7th lvl of the mines.
(Thanks to Ben Reynolds confirming the boss is on the 8th level 'Overseers Chamber'.)
After defeating him you will talk to one the main characters for a short minute before moving deeper into the mines. Also after killing the Overseer you will get an item called the Overseer's Eye. This item will permanently raise your health by about 20 - 25 points.