Boreal Outrider Knight

Fear Aura. Any creature hostile to the Boreal Outrider Knight that starts its turn within 20 ft. of the Boreal Outrider Knight must make a DC 21 Wisdom saving throw, unless the Boreal Outrider Knight is incapacitated. On a failed save, the creature is frightened until the start of its next turn. If a creatures saving throw is successful, the creature is immune to this effect until the start of the next round.

Flurry of Steel. The Boreal Outrider Knight can make a free attack against any creature that enters its threat range. Even if a creature disengages, the Boreal Outrider Knight can still make an attack of opportunity if it enters or leaves its threat range.

Magic Resistance. The Boreal Outrider Knight has advantage on saving throws against spells and other magical effects.

Magic Weapons. The Boreal Outrider Knight's weapon attacks are magical.

Multiple Attack. The Boreal Outrider Knight makes 4 attacks, split between its Twirl and Irithyll Straight Sword. 

Frost Thrust. All creatures in a 30 ft. long, 10 ft. wide, line must immediately make a DC 21 Dexterity saving throw suffering 258 (12d8) cold damage on a failure or half as much damage on a successful saving throw.

Twirl. Melee Weapon Attack: +15 to hit, reach 15 ft., all creatures in reach. Hit: 22 (4d6 + 8) slashing damage plus 27 (6d8) cold damage. Roll separately for each creature in reach.

Irithyll Straight Sword. Melee Weapon Attack: +15 to hit, reach 5 ft., 1 target. Hit: 22 (4d6 + 8) slashing damage plus 27 (6d8) cold damage.

Blood Soaked. This creature spends Health Points instead of Legendary points to use Legendary Actions.

Bloody Presence. Upon reaching 150 health, all creatures within 20 ft. of Boreal Outrider Knight must make a DC 21 Strength saving throw. On a failure, the creature suffers 24 (3d10 + 8) bludgeoning damage and 27 (6d8) necrotic damage. On a successful saving throw, the creature suffers half damage.

Blade Dance. (Spend 5 Health) Boreal Outrider Knight moves 10 ft. after each attack it makes for the rest of the round. This movement does not provoke attacks of opportunity.

Ash Cloud. (Spend 5 Health) All creatures within a 20 ft. radius of Boreal Outrider Knight must immediately make a DC 21 Constitution saving throw. On a failure, the creature is [effect]stunned[/effect] until the end of their next turn. On a successful saving throw, the creature instead suffers 27 (6d8) poison damage.

Unrelenting Slashes. (Spend 5 Health) Each creature the Boreal Outrider Knight hits with a melee attack suffers an additional 1d8 piercing damage at the start of their turn. The extra damage ceases when the creature succeeds on a DC 12 Wisdom (medicine) check or receives magical healing.