Gisa, Glorious Resurrector

If a creature an opponent controls would die, exile it instead.
At the beginning of your upkeep, put all creature cards exiled with Gisa, Glorious Resurrector onto the battlefield under your control. They gain decayed. (A creature with decayed can't block. When it attacks, sacrifice it at end of combat.)

Swamp

Necrosavant

{3}{B}{B}, Sacrifice a creature: Return Necrosavant from your graveyard to the battlefield. Activate only during your upkeep.