Requirements for this acheivement are located in Hatred's Shadow area (it's the one after Mines of Avarice) Right after you meet the guy who gives you quests about Ninja Looters, you go right from exit of his cave then go forward, pass the entrance to "The Pit". And after that on the left side there will be a big black space when you put your crosshair on it it will say "The Darkness" simply kill it to obtain the achievement.