Large Fiend, neutral evil
Armor Class 13 (natural armor)
Hit Points 68 (8d10+24)
Speed fly +90, walk +60
Challenge 3
Damage Immunities fire
Senses passive Perception 11
Languages understands Abyssal, Common, and Infernal but can't speak
Confer Fire Resistance. The nightmare can grant resistance to fire damage to anyone riding it.
Illumination. The nightmare sheds bright light in a 10-foot radius and dim light for an additional 10 feet.
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.
Source: 5e Core Rules · OGL/Open