Reaper's Hourglass

Reaper's Hourglass costs {1} less for each creature
card in your graveyard.
Whenever a creature dies, each player puts the
top X cards of their library into their graveyard,
where X is the creature's power.
{12}, Exile Reaper's Hourglass: Each player exiles
their library, then shuffles their graveyard into
their library.