Per usual, the quest line begins with an NPC warlock near the trainers in one of each faction's major cities. When you reach level 10 as a warlock, go to any warlock training area for your faction (other than the level 1-6 starting areas) and there will be a quest for you, either starting you on the quest or directing you to the right person to talk to.
For the Horde, Ophek in Razor Hill will send you to Gan'rul Bloodeye in Orgrimmar, and Ageron Kargal in Brill will send you to Carendin Halgar in The Undercity.
Alliance warlocks will be directed to Gakin the Darkbinder in Stormwind, either by Remen Marcot of Goldshire or Lago Blackwrench of Ironforge (to whom you may be directed by the gnome warlock trainers of Kharanos).
In either case, the quest giver will send you to retrieve a quest item from someone nearby, who will not part with it willingly, and when you return with it, the ritual begins. A trainer will then tell you to summon a Voidwalker somewhere nearby. You must defeat it to earn the power to summon one yourself again in the future. Others can assist, of course.
The blood elf quest requires you to fight your way into a village in the Ghostlands guarded by Quel'dorei Ghosts and Wraiths because there isn't a summoning circle in Silvermoon City. All in all, this quest is far more difficult than the other Horde versions.