You magically summon a Nightmare, which draws strength from your bond with Myrkul. The Nightmare is friendly to you and your companions and obeys your commands. Its stat block uses your proficiency bonus (PB) in several places. The Nightmare bears necromantic markings, indicating its arcane origin.
In combat, the Nightmare acts during your turn. It can move, attack and use its reaction on its own. If you are incapacitated, the Nightmare can take any action of its choice. The Nightmare also vanishes if you die.
When you finish a long rest, you can re-summon the Nightmare and it appears in an unoccupied space within 5 feet of you.
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.
Undying Bond. You can add your proficiency bonus to any ability check or saving throw that the Nightmare makes.
Unusual Nature. The Nightmare doesn’t require air, food, drink, or sleep.
Hooves. Melee Weapon Attack: +(STR+PB) to hit, reach 5 ft., one target. Hit: (2d8) + (STR+PB) bludgeoning damage plus (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. The Ethereal plane is not stable at this time, some sages even say that the Ethereal plane does not exist any longer since the Second Sundering.
A Nightmare appears in a cloud of roiling smoke, its mane, tail, and hooves wreathed in flame. The creature’s unearthly black form moves with supernatural speed, vanishing in a cloud of brimstone as quickly as it appeared.
Dread Steed. Also called a “demon horse” or “hell horse,” the Nightmare serves as a steed for creatures of exceptional evil, carrying demons, devils, death knights, liches, night hags, and other vile monsters. It resembles a fiendish horse, and a Nightmare’s fiery red eyes betray its malevolent intelligence.
A Nightmare can be summoned from the Lower Planes, but unless a worthy sacrifice is offered to it as food upon its arrival, the Nightmare displays no special loyalty to the creature it serves.
Creating a Nightmare. Nightmares don’t appear naturally in the multiverse. They must be created from Pegasi. The ritual that creates a Nightmare requires the torturous removal of a Pegasus’s wings, driving that noble creature to evil as it is transformed by dark magic.