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.
![Crystal Keeper [SDCB-EN010] Common](http://www.cardmerchanttakapuna.co.nz/cdn/shop/products/a2f04399-9b33-5268-a257-f21600f98a18.png?v=1698526545&width=1445)