If a creature's power is less than 0 when it's doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is
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This card doesn't do much over the course of the game, but it's got acceptable stats for the mana value and it has an incredibly potent ETB effect if you can pay the cost
It would go to the graveyard and the next card would be turned face up and be in play (assuming it's a permanent)
And, even if the opponent isnt completely wiped out right away, its a source of continuous damage
It has one of the strongest colorless ETB effects in the game, giving you protection the turn you cast it, so you get a nice advantage even if you cast this card in the late game