Diabolic Ritual

Spending a Soul Shard on a damaging spell grants Diabolic Ritual for 20 sec. While Diabolic Ritual is active, each Soul Shard spent on a damaging spell reduces its duration by 1 sec.

When Diabolic Ritual expires you gain Demonic Art, causing your next Hand of Gul'dan to summon an Overlord, Mother of Chaos, or Pit Lord that unleashes a devastating attack against your enemies.