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.
Actions
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.
RollQuest summary Senses: passive Perception 11. Languages: understands Abyssal, Common, and Infernal but can't speak. Speed: 60 ft., fly 90 ft.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.