Soulshatter is another new skill that warlocks receive, this one at level 66. As a variation on the usual "Spells cost mana" theme, this spell costs you 8% of your base health. Base health is your health before any bonuses from items, so it's probably between 3 and 4 percent of your hit points. It also costs one Soul Shard. For the warlocks this spell is great because we didn't have a practical way to dump aggro before, and now we can easily reduce our produced threat to stay below the threat of our tank. The 5 minute cooldown means it's only usable once or maybe twice per fight, but that's usually enough to keep you far enough down the aggro list to not be a threat.
Sorry, no action shot for this one. Trying to capture an instant-cast spell with the screenshot tool would run me out of shards fast.