Armor Class 13 (natural armor)
Hit Points 68 (8d10 + 24)
Speed 60 ft., fly 90 ft.
Damage Immunities fire
Senses passive Perception 11
Languages understands Abyssal, Common, and Infernal but can't speak
Challenge 3
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.
Monster Manual (2014), p. 235