Traits

Confer Fire Resistance: Anyone riding the nightmare monster could get the fire damage resistance.

Illumination: This nightmare casts bright light within a 10-foot area and also dims for an additional 10 foot.

Actions

Hooves: This hooves action by the dnd nightmare monster can make a Melee weapon. Attack: +6 to hit. Reach 5 feet., one target. Hit: 13 (2d8 + 4) bludgeoning damage plus 7 (2d6) fire damage.

Ethereal Stride: It is possible for up to three willing creatures within 5 feet of the nightmare to enter the Ethereal Plane by simply walking on a material plane or vice versa.

Attributes of Nightmare DnD 5E Monster

AC 13 (Natural Armor).
Alignment Neutral Evil
CHA 15
CON 16
Challenge Rating 3
DEX 15
HP 68 (8d10+24).
INT 10
Immunities Fire
Languages Although he can understand the Abyssal, Common and Infernal, he cannot speak.
Passive Perception 11
Roll 0 Hooves 1d20 + 6 d8+4+2d6
STR 18
Size Large
Speed 60 ft. Fly 90 ft.
Type fiend
WIS 13