Summoner's Cube

{2},{t}: Flip a coin. If you win the flip, sacrifice
Summoner's Cube. Search your library for a
creature card and exile it. You may cast that
card and you may spend mana as though it were
mana of any color to cast that spell.
"A Cenobite Puzzle Box has infinite solutions, each
allowing the summoning of a single creature.
It does not allow anything to be sent back.