Art of summoning

At the beggining of your upkeep, mill four cards from the top of your library then roll a d20.
[1-10] create X number of 2/2 zombies with decade, where X is the number of creatures discarded this way.
[11-19] Create a copy of each creature discarded this way this turn.
[20] Return every creature card from your graveyard onto the battlefield and create a copy of every creature that entered this way.