Doomsday Clock

Doomsday Clock enters with 12 Doomsday counters.
At the beginning of each player's upkeep remove a Doomsday counter from Doomsday Clock.
When the last Doomsday counter is removed, sacrifice Doomsday Clock and exile all nonland permanents in play.
{4},{t}: Add or remove a Doomsday counter from Doomsday Clock.
“The end was never sudden; it was scheduled.”