Flying (This creature can't be blocked except by creatures with flying or reach.)
Nightmare's power and toughness are each equal to the number of Swamps you control.
4/1/2008 If you control 0 swamps, then the Nightmare has 0 toughness and will be put into its owner’s graveyard as a state-based action right before the next player gains priority.
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.