Horde Warlock

Demonology — {2}, {t}: Search your library for a Demon, Devil or Imp creature card, reveal that card and put it into your hand.

Drain — {3}, {t}: Deal 2 damage to target creature. Whenever a creature dealt damage this way would die exile it instead and put it face down into play under your control. It is a colorless artifact named "Soul Stone" with the ability "{t}: Sacrifice this artifact, reduce the cost of the next creature you cast or activated ability this turn by {2}."