Crystal Keeper (FLOD-EN093)

Pendulum Effect: Each turn, the first time an "Ultimate Crystal" monster(s) and/or "Crystal Beast" card(s) you control would be destroyed by card effect that turn, it is not destroyed.

Monster Effect: If a "Crystal Beast" monster you control battles an opponent's monster, during damage calculation (Quick Effect): You can Tribute this card from your hand or face-up from your field: your battling monster's ATK/DEF become double its original ATK/DEF during that damage calculation only, but it is destroyed at the end of this Damage Step.