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At the beginning of each opponent's upkeep, that player reveals the top card of their library. If it's a creature card, the player puts the card into their graveyard and Shapeshifter's Marrow becomes a copy of that card. (If it does, it loses this ability.)
If the revealed card is a creature card, Shapeshifter’s Marrow becomes a copy of that card permanently. It’s no longer an enchantment.
If the revealed card isn’t a creature card, it stays on top of that player’s library. Shapeshifter’s Marrow remains an enchantment.
In a Two-Headed Giant game, this ability triggers separately for each player at the start of the opposing team’s turn. If the first ability to resolve reveals a creature card, Shapeshifter’s Marrow becomes a creature. If the second ability to resolve also reveals a creature card, the ex-Shapeshifter’s Marrow then becomes a copy of that creature.
