Bring an Infernal Orb to Tabetha in Dustwallow Marsh.
The first step is to acquire an orb with power. The warlocks of the Burning Blade have nice, high quality orbs... it's too bad they've bound demons into them! But we can fix that, yes we can.
Go to Desolace and snatch an infernal orb from a Burning Blade summoner. You'll find summoners at the Mannoroc Coven.
When you have the orb, return to me.