Large Fiend
Armor Class 13
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
Evil. The nightmare radiates an Evil aura.
Fiery Hooves. The nightmare sheds bright light in a 10-foot radius and dim light for an additional 10 feet. The nightmare leaves charred hoofprints.
Fire Resistance. The nightmare can grant fire resistance to a rider.
Hooves. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) bludgeoning damage plus 7 (2d6) fire damage. If the horse moves at least 20 feet straight towards the target before the attack the target makes a DC 14 Strength saving throw falling prone on a failure. The nightmare can move through the space of a prone creature as long as it does not end its turn there.
Ethereal Shift (Recharge 5-6). The nightmare and a rider magically pass from the Ethereal Plane to the Material Plane or vice versa.
Source: Level Up Advanced 5e Monstrous Menagerie · OGL/Open