Doors of death

Whenever a card that you control is sent to the graveyard, put 3 time counters on it.
At the beginning of your upkeep, take one time counter off every creature that has one.
When a creature you control has no time counters on it, return it back to the battlefield.
(this counts as casting it.)
If you control Thanatos, chained death, when a creature you control is put in a graveyard, immediately put it back on the battlefield.
(This counts as casting it.)