Nightmare | Magic: The Gathering
Formats Nightmare is Legal in
Official Oracle Text for Nightmare
Nightmare's power and toughness are each equal to the number of Swamps you control.
Official Rulings for Nightmare
10/1/2009 : Nightmare's power and toughness changes as the number of Swamps you control changes.
7/1/2013 : The ability that defines Nightmare’s power and toughness works everywhere, not just on the battlefield.
7/1/2013 : Nightmare’s ability counts all lands you control with the subtype Swamp, not just ones named Swamp.