You must reveal exactly one card to cast Charge of the Forever-Beast; you can’t cast it without revealing a card, and you can’t reveal additional cards. The revealed card remains revealed until Charge of the Forever-Beast resolves.

As Charge of the Forever-Beast resolves, it deals damage based on the revealed card’s power if it’s still in your hand. If that card has left your hand, Charge of the Forever-Beast deals damage based on the card’s power immediately before it left your hand.