Nuclear Warhead

At the beginning of each upkeep, add a
countdown counter to Nuclear Warhead.
Once 5 countdown counters have been added
destroy all creatures opponents control.
During an opponents turn they may spend mana towards Nuclear Warhead's mana pool. If 20
mana is spent this way, destroy Nuclear Warhead.
(Mana spent towards this mana pool is cumulative
between opponents and does not reset.)