Undersea Ruins (Aquapolis 138/147)

Rules

This card stays in play when you play it. Discard this card if another Stadium card comes into play.

Once during each player's turn (before attacking), that player may flip a coin. If heads, that player chooses 1 of his or her Evolved Pokémon in play and discards the top Evolution card from that Pokémon, devolving it.