1 Apr 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.
1 Oct 2009
Nightmare’s power and toughness changes as the number of Swamps you control changes.
1 Jul 2013
The ability that defines Nightmare’s power and toughness works everywhere, not just on the battlefield.
1 Jul 2013
Nightmare’s ability counts all lands you control with the subtype Swamp, not just ones named Swamp.