Cthulhu, Sleeper of R'lyeh

Flying, Trample, Indestructible.
Cthulhu, Sleeper of R'lyeh is not a creature unless each player has 20 or more cards in their graveyard.
At the beginning of your upkeep, each player puts the top 10 cards of their library into their graveyard, then you may exile target card in a graveyard and place a Dream counter onto it. You may play it from exile for as long as it remains exiled and you may use mana as though it were mana of any kind to cast the spell.