All "Crystal Beast" monsters on the field and in the Graveyard become DARK. If "Rainbow Dragon" or "Rainbow Dark Dragon" attacks, negate the effects of the attack target during that Battle Phase. During damage calculation, if a "Crystal Beast" monster you control battles and you would take damage: You can send 1 "Crystal Beast" monster from your Deck to the Graveyard; you take no Battle Damage from that battle.
How to Get
Sends from your Deck to your Graveyard for cost
Negates the effects of your opponent's Effect Monsters