quoting wowhead:
Dark Simulacrum | 20 Runic Power | 40 yd range | Instant | 1 min cooldown.
Places a dark ward on an enemy that persists for 8 sec, triggering when the enemy next spends mana on a single-target spell, and allowing the Death Knight to unleash an exact duplicate of that spell. Against nonplayers, only absorbs some harmful spells (Wowhead).