Promise, The Lost City Weapon

When Promise enters the battlefield, put X charge counters on it.
{x}{t}: Remove X charge counters from Promise and deal
X damage to target player and each creature that player controls. You can only activate this ability when Promise is a creature. X cannot equal more than the crewing creature's power.
Crew 1 (Tap any number of creatures you control with total power 1 or more: Promise becomes a creature until end of turn.)
Promise can only be crewed by one creature.