Underdark Knight

Innate Spellcasting. The Underdark Knight's spellcasting ability is Wisdom (+6, DC 16). They can cast the following spells at will, requiring no material components:

Unlimited: [Hellish Rebuke]

Cold Soul. The Underdark Knight has a cold aura constantly emanating from it. Creatures that start their turn within 5 feet of the Knight take 1d6 cold damage, Heat Metal has no effect, and cold damage heals the Knight for half the damage it took.

Evasion. When the Underdark Knight succeeds on a Dexterity Saving Throw against an attack that will only deal half damage to a creature that has made a successful Dexterity saving throw, the attack will instead deal no damage.

Cooled Armor. When Investiture of Ice is active on the Underdark Knight, they are immune to the spell Heat Metal.

Multiattack. The Underdark Knight can attack twice with their Helix Sword: Split attack or once with their Helix Sword: Whole Attack.

Helix Sword: Split. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 10 (1d10+5) slashing damage. 

Helix Sword: Whole. Melee Weapon Attack: +8 to hit, reach 5 ft., 1 target. Hit: 25 (4d10+5) slashing damage.

Roar (Recharge 5-6). The Underdark Knight lets out an inhuman roar. All creatures wihin 60 ft. must make a Wisdom saving throw DC 16 and on a failed save will become frightened for one minute. The frightened creature may reroll the saving throw on the start of each of their turns, and upon success will no longer be frightened as well as gain immunity to fright for one minute.

Swift. Once per round, the Underdark Knight can use the Disengage Action as a bonus action.

The Underdark Knight can make up to two Legendary Actions per turn. They can only use one at a time and only at the end of another creature's turn. These Legendary Actions reset at the start of the Underdark Knight's next turn.

Helix Sword: Split (Costs one action). The Underdark Knight makes one Helix Sword: Split attack.

Helix Sword: Whole (Costs two actions). The Underdark Knight makes one Helix Sword: Whole attack.