As far as I am aware, the Aspect Warlock is a class introduced by the fan writer Greywolf in his non-canon addition to the Warcraft RPG called "Even More Magic & Mayhem." Unfortunately I no longer have a copy and his website no longer hosts downloads for the PDF so I can't verify that. What I do know for certain is that it's not mentioned in any canon source I checked.
That being said, the ingame talent Grimoire of Sacrifice seems to get close to what you're looking for. You sacrifice a demon and temporarily gain some of its abilities.
That being said, the ingame talent Grimoire of Sacrifice seems to get close to what you're looking for. You sacrifice a demon and temporarily gain some of its abilities.