Bucephalus

Confer Fire Resistance. Bucephalus can grant resistance to fire damage to anyone riding him.

Dark Illumination. Bucephalus sheds bright light in a 10-foot radius and dim light for an additional 10 feet. This effect neutralizes the areas of spells that create sunlight in the spaces it overlaps, and creatures within Bucephalus's affected radius do not suffer effects of sunlight, though they are still affected by instantaneous spells like sunburst.

Eternal Servitude. When Bucephalus is reduced to 0 HP, he explodes in a fiery burst. All creatures within 10 feet of him must make a DC 16 Dexterity saving throw, taking 13 (3d8) fire damage on a failure and half as much on a success. Flammable objects that are not worn or carried in the area are ignited. Bucephalus can be summoned again by Strahd or Carhana after 24 hours, though they must spend 10 minutes doing so.

Trample. If Bucephalus move at least 20 feet in a straight line towards a creature and then hits with an attack with his hooves, that target must succeed on a DC 16 Strength saving throw or fall prone.

Multiattack. Bucephalus makes two attacks with his hooves.

Hooves. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 15 (2d10 + 4) bludgeoning damage plus 9 (2d8) fire damage.

Ethereal Stride. Bucephalus and up to three willing creatures within 5 feet of him magically enter the Border Ethereal from the Material Plane, or vice versa. While in the Border Ethereal, they are affected as if by etherealness, though they cannot perceive or be perceived by or affect or be affected by creatures on the Ethereal Plane, and cannot stay for longer than 8 hours.