Professor Ravendark

Vigilance, lifelink.
Every time a non-token creature arrives on the
battlefield under your control, if you've cast it
from your hand, sacrifice it then summon a 2/2
student token.
{w}{b}, Sacrifice a student: Professor Ravendark
gain indestructible until end of turn.
{w}{w}{b}{b}, Sacrifice three students: return target
creature from your graveyard to the battlefield
engaged.