Emn, the Ancient Lifeoak

Constant Upkeep {g}(At the beginning of your
upkeep, if this permanent is on the battlefield, put an
age counter on this permanent. Then you may pay {g}. If you don't, sacrifice it.)
Defender
At the beginning of your end step, if Emn, the
Ancient Lifeoak has 15 or more age counters on
it, you win the game.