Defeated Kanrethad Ebonlocke and gained command over fel energy before the expedition to Draenor set out.
Title Reward: <Name> of the Black Harvest
Going from the FoS's current tool-tip ("Defeat Kanrethad Ebonlocke and gain command over fel energy as a level 90 warlock."), this probably means that any warlock post-Mists will still be able to earn it as long as they're level 90. I would not be surprised if a later iteration of this achievement adds a maximum item level restriction. (See Herald of the Titans.) Kanrethad Ebonlocke is a nasty piece of work, and warlocks rightly take pride in overcoming him.
Edit: Build 18379 has changed the tooltip to explicitly note that the achievement must have been earned prior to the "expedition to Draenor." /sigh. Well, guess this means I have to forgo trying for Herald of the Titans on my warlock. (ノಠ益ಠ)ノ彡┻━┻