Avernus House Knight

Legendary Resistance (3/Day). If the knight fails a saving throw, he can choose to succeed instead.

Innate Spellcasting. The knight counts as a 17th level caster, and casts one of the following spells, requiring no material components and using Wisdom as the spellcasting ability (spell save DC 18, +10 to hit with spell attacks):
At will: chill touch, comprehend languages, fog cloud, mage hand, toll the dead

3/day each: hold person, identify, scrying, speak with dead

1/day each: animate dead, bestow curse, finger of death

The Deadlights of Avernus. The knight's connection to the might of avernus has grown to the point that his spellcasting has been greatly enhanced, such that their very spells seem tainted by the realm. Any spells cast by the knight that do necrotic damage ignore a targets resistances, and the knights bestow curse instead has a range of 30 ft.

The Mists of Avernus. The mists of avernus gather about the knight at all times, clinging to them and empowering them and giving them a supernatural sight. The mists reach out in a 60 ft circle, centred on the knight. Creatures within the mists cannot benefit from half cover or three-quarters cover when attacked by the knight, and the knight is always aware of any creature within the radius of the mist, regardless of whether they are visible or invisible. Additionally, any illusions within the radius of the mist are seen for what they are by the knight.

Borrowed Time (1/day). When the knight is reduced to 10 hp or lower, but not below 1 hp, the knight pulls upon the might of avernus to allow it to linger for a while long. Lasting until the end of the knights next turn, any damage that the knight would take, is instead converted into health, healing them by the amount of damage they would have taken. The knight cannot be healed beyond their hit point maximum.

Multiattack. The knight makes two attacks with his hell pike or his avernus greatsword. 

Hell Pike. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 26 (4d10 + 7) piercing damage.

Avernus Greatsword. Melee Weapon Attack: +13 to hit, reach 5ft., one target. Hit: 28 (6d6 + 7) slashing damage, and the target must make a DC 19 Constitution saving throw. On a failure, their speed is reduced by half until the start of the knights next turn.

Aphotic Shield. As an action, the knight conjures up a shield of pure energy from the very waters of the river styx, surrounding himself or another target within 30 ft. While the shield exists, it absorbs any damage that would be intended for the target, and moves with them. Once it absorbs up to 80 points of damage, the shield explodes outwards. Every creature within 10ft of the shield, except for the target that was being shielded, must make a DC 17 Dexterity saving throw, taking 45 (10d8) force damage on a failed save, or half as much on a success.

Curse of Avernus. As a bonus action, the knight can point at a single creature within the radius of the Mists of Avernus, and curse them. The curse lasts until the end of the knights next turn. While cursed, all attacks against the creature are at advantage, and the creatures attacks are at disadvantage.

The knight can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The knight regains spent legendary actions at the start of his turn.

Avernus Casting. The knight casts a single spell from their at will spellcasting.

Avernus Greatsword. The knight makes an Avernus Greatsword attack.

Mists Coil (Costs 2 Actions). The knight does 10 damage to themselves, and then points to a target other than themselves within the radius of the Mists of Avernus. That target must make a DC 17 Dexterity saving throw, taking 21 (6d6) necrotic damage on a failed save, or half as much on a success, and the knight regains health equal to half of the damage taken by the target. Alternatively, if the target is an ally of the knight, they forgo the save and are instead healed by 21 (6d6) hit points, and the knight does not regain any health.