Welcome to Segovia

You may cast spells by paying X instead of their
mana costs, where X is the greatest power among creatures you control. (You must have a creature.)
Whenever a creature attacks you or a
planeswalker you control, that creature has base
power X until end of turn. It loses all counters.
Whenever a creature you control attacks, it has
base power and toughness equal to the greatest
power among creatures your opponents control
until end of turn.