Forced synergy with
exactly one other card in game is almost always impractical at best (most players will prefer to use the card slot for more flexible cards like Drain Life or Steal), as ET mentioned above.
How about having it trigger whenever you Drain HP or Drain Quanta? It keeps the drain synergy, but improves on it to work with more cards in
's arsenal (Drain Life, Nightmare, Vampire Dagger, Pest), as well as a few cards outside of Darkness (Black Hole, Flooding).