Nightmare
Creature — Nightmare Horse
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.
The Nightmare arises from its lair in the swamps. As the poisoned land spreads, so does the Nightmare's rage and terrifying strength.
Melissa A. Benson
Rulings
Nightmare’s ability counts all lands you control with the subtype Swamp, not just ones named Swamp.
(wotc • 2013-07-01)
The ability that defines Nightmare’s power and toughness works everywhere, not just on the battlefield.
(wotc • 2013-07-01)
Nightmare’s power and toughness changes as the number of Swamps you control changes.
(wotc • 2009-10-01)
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.
(wotc • 2008-04-01)
Rulings
Nightmare’s ability counts all lands you control with the subtype Swamp, not just ones named Swamp.
(wotc • 2013-07-01)
The ability that defines Nightmare’s power and toughness works everywhere, not just on the battlefield.
(wotc • 2013-07-01)
Nightmare’s power and toughness changes as the number of Swamps you control changes.
(wotc • 2009-10-01)
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.
(wotc • 2008-04-01)