The Heron Moon is a rare Legendary Land card from Mystery Booster 2 (MB2), collector number 612. Current market price: $0.30, TCGplayer market prices via Scryfall.
{T}: Add {C}. {1}, {T}: Exile the bottom card of target opponent's library. Whenever one or more cards an opponent owns are put into exile, put a release counter on The Heron Moon. Then if it has thirteen or more release counters on it, sacrifice it and create a copy of Emrakul, the Promised End. Cast it without paying its mana cost.