WoW DK question dark simulacrum?

You apply the instant debuff which lasts for 8 seconds on the target, during which any spell the target casts that costs mana will then appear in your action bar where your dark simulacrum ability is. You have 20 seconds to unleash the spell, which will scale with your attack power. It won't interrupt or silence and the spell HAS to cost mana, ie non-mana users like hunters, rogues, other dks can't be copied.