Affliction Tome: Drain Soul II

"Drains the soul of the target, causing Shadow damage over If the target is at or below 25% health, Drain Soul causes four times the normal damage. If the target dies while being drained, and yields experience or honor, the caster gains a Soul Shard. Each time the Drain Soul damages the target, it also has a chance to generate a Soul Shard. Soul Shards are required for other spells."