Phoenix Heart is an UPGRADE.
For this card to be played there has to be a creature on the board.
Phoenix Heart can be played on friendly or enemy creatures.
When Phoenix Heart is attached to a creature, that creature gains Destroyed: Return this creature to its owner's hand and deal 3 damage to each creature in play.
When the creature that Phoenix Heart is attached to leaves play for any reason, Phoenix Heart goes to its' owners discard pile.