Grey, Eternal Youth

When Grey, Eternal Youth deals combat damage
to a creature, that creature doesn't untap during
its controller's next upkeep.
When Grey, Eternal Youth becomes untapped,
you gain X life and target player loses X life,
where X is equal to the number of tapped
creatures that player controls.
"My mother wanted me to be a weapon, for all of
eternity. I choose to be a scholar, instead."