Drain Psyche

Necro {x}{b}(When you cast this spell, you may
remove a creature card with {x}{b} in its casting cost
from your graveyard. If you do, this spell costs {x}{b} less to cast, with X rounded down to the nearest
even number.)
Target opponent reveals their hand and exiles a
card with a converted mana cost of X or less.
Until the end of your next turn, you may cast
that card without paying its mana costs. If you do
you gain X life, and that opponent loses X life.