Soul Preserver

Flying
You may exile target creature you control as you cast Soul Preserver. If you do, Soul Preserver costs X less to cast and enters the battlefield with
X -1/-1 counters on it where X is that creature's converted mana cost.
{w}{w}{w}, Sacrifice a creature: You may remove a
-1/-1 counter from Soul Preserver. If you do,
you gain 3 life.