Death and Decay is a targetable AoE Death knight ability learned at level 60. It is the key to AoE tanking, and should always be used on cooldown when facing two or more mobs. Against single targets, do not use it, as your other attacks provide more threat. For DPS, it is worth using when facing large packs. The bonus threat should not be enough to draw aggro with a competent tank, provided that you are not in [Blood Presence].