Orkow, Of Lifes Cycle

At the beginning of your pre-combat main phase, you may sacrifice a creature. If you do, add {b}{g}
At the beginning of your upkeep if you don't control a spirit token, exile a creature card from a graveyard under Orkow, Of Lifes Cycle and create a 0/1 white and black spirit creature token with "This creature enters the battlefield with 2 rebirth counter." And "At the beginning of your end step remove a time counter from this creature. When this creature has no rebirth counters return target card exile