The Giphuktagon

The Giphuktagon enters the battlefield with X
doom counters on it.
At the beginning of each player's end step, or
whenever a player casts a spell, that player rolls a
d20. If the result is less than the number of
doom counters on The Giphuktagon, remove all counters from it and that player mills 1 for each counter removed this way. Otherwise, put a
doom counter on it. Then, if it has twenty doom
counters on it, that player wins the game.