Ghost Gym

When another gym enters the battlefield, exile
this with a visited counter on it. When you
control no gyms, if this is in exile with a visited
counter on it, you may put this in your graveyard.
At the beginning of that player's upkeep they
may return a creature from their graveyard to
their hand. If they have 7 or more gyms in exile
they may put it on the battlefield instead.