Hanxua, the Lady of Balance

If a player other than you controls the creature with the greatest power, creatures that player controls get -1/-1.
Whenever a creature you control blocks a creature with greater power, it gets +1/+1 and gains lifelink until end of turn.
"I have watched empires rise and fall in the blink of an eye. This universe seeks balance; nothing more, nothing less. It will never care about you."