If I summon a creature like Boneyard Wurm whose power and toughness depends on the number of creatures in my graveyard, and afterwards another creature of mine dies, do the Boneyard Wurm's power and toughness increase? What about the opposite: if a creature card is removed from my graveyard, do its power and toughness decrease?
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1I picked a card that has power and toughness depending on number of creature cards in your graveyard as you described - pretty sure it's what you had in mind, but let us know if you had something different in mind.– CascabelCommented Jan 26, 2016 at 7:18
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To get really technical P/T of a creature is only checked as a creature's state. This means that if a spell says remove all creatures from your graveyard and put back 4 of them. While The Wurm will be 0/0 for a period during spell resolution it does not die because the p/t will be 4/4 when it is checked. See 704.3 in CR– HoganCommented Jan 26, 2016 at 15:34
1 Answer
Yes, that creature's power and toughness will change as the number of creatures in your graveyard changes. It doesn't say something weird like "when it enters the battlefield, count the creatures in your graveyard, etc etc". It just says:
Boneyard Wurm's power and toughness are each equal to the number of creature cards in your graveyard.
So that's what it does. However many creature cards are in your graveyard right at this moment, that's its power and toughness. Add one, they go up by one, remove one, they go down by one.
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7Also worth noting is that / creatures have that power and toughness in any zone, so if, for example, an effect allows you to search your library for a creature card with power >= 4, and you have >= 4 creatures in your graveyard, you could select Boneyard Wurm. Commented Jan 26, 2016 at 8:02