Svogthos, the Restless Tomb
Land
: Add 
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: Until end of turn, this land becomes a black and green Plant Zombie creature with "This creature's power and toughness are each equal to the number of creature cards in your graveyard." It's still a land.
Martina Pilcerova
Rulings
Activating the ability that turns it into a creature while it’s already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.
(wotc • 2009-10-01)
A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.
(wotc • 2008-08-01)
While animated, Svogthos’s power and toughness changes each time a creature card enters or leaves its controller’s graveyard.
(wotc • 2005-10-01)
Rulings
Activating the ability that turns it into a creature while it’s already a creature will override any effects that set its power and/or toughness to a specific number. However, any effect that raises or lowers power and/or toughness (such as the effect created by Giant Growth, Glorious Anthem, or a +1/+1 counter) will continue to apply.
(wotc • 2009-10-01)
A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.
(wotc • 2008-08-01)
While animated, Svogthos’s power and toughness changes each time a creature card enters or leaves its controller’s graveyard.
(wotc • 2005-10-01)