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Large fiend, neutral evil
Confer Fire Resistance. The nightmare can grant resistance to fire damage to anyone riding it.
Hooves. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 7 (2d6) fire damage.
Ethereal Stride. The nightmare and up to three willing creatures within 5 feet of it magically enter the Ethereal Plane from the Material Plane, or vice versa.
A nightmare appears in a cloud of roiling smoke, its mane, tail, and hooves wreathed in flame. The creature's unearthly black form moves with supernatural speed, vanishing in a cloud of brimstone as quickly as it appeared.
Dread Steed. Also called a "demon horse" or "hell horse," the nightmare serves as a steed for creatures of exceptional evil, carrying demons, devils, death knights, liches, night hags, and other vile monsters. It resembles a fiendish horse, and a nightmare's fiery red eyes betray its malevolent intelligence.
Creating a Nightmare. Nightmares don't appear naturally in the multiverse. They must be created from pegasi. The ritual that creates a nightmare requires the torturous removal of a pegasus's wings, driving that noble creature to evil as it is transformed by dark magic.