Stagnant Morass

Stagnant Morass enters the battlefield tapped.
At the beginning of your upkeep, put a season
counter on Stagnant Morass. Then, if there are
four or more season counters on it, remove all counters from it. For each counter removed this
way, exile up to that many cards from graveyards.
As long as Stagnant Morass has no season
counters on it, it loses all other activated abilities
and gains: "{t}: Add {s}."