I'm currently running a Void Warlock and I and many others routinely use Devour and Child of the Old God's which drain life from and weaken enemies. I'm sorry, but that sounds almost identical to the Nezarec's Curse as Eido describes it. I believe Nezarec has a stronger connection to the Void that he does to the darkness. Perhaps he was even born of it. Hence why we have the Nezarec's Sin helmet which recharges our void abilities. It's possible that Nezarec created the Abyssal Extractors to control or maximize the effect of his life draining abilities or his "curse". It's possible that Nezarec can't control this and its just intrinsic to his being. He's hungry like the Void...like a Void Soul incarnate.