Azurnith

Spellcasting. Azurnith is a #-level spellcaster. Its spellcasting ability is Charisma (spell save DC 25, +17 to hit with spell attacks). Azurnith has following Paladin spells prepared:
1st level (7 slots): Divine Smite
2nd level (5 slots): Locate Object, Prayer of Healing
3rd level (3 slots): Revivify
4th level (2 slots): Banishment
5th level (1 slot): Geas

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

Aura of Oaths (Divine Aura). Within 600 ft. of Azurnith, creatures who willingly break a promise, deal, or bond they have sworn they have disadvantage on attack rolls until the end of their next turn. Azurnith is aware of every broken vow within this range.

Keeper of Vows. Whenever a creature triggers Aura of Oaths within 60 ft, Azurnith regains 30 hit points and his next attack roll is made with advantage.

Multiattack. Azurnith makes three attacks with his Blade of the Oathbound, or two with his blade and one use of Oathbreaker’s Command.

Blade of the Oathbound. Melee Weapon Attack: +14 to hit, reach 10 ft., one target.
Hit: 27 (4d8+9) slashing damage plus up to 12 psychic damage if they have broken a vow in the last year, scaling per month.

Oathbreaker’s Command (Recharge 5 turns). Azurnith targets one creature within 60 ft. who has broken a vow (even minor) within the last 24 hours. That creature must succeed on a DC 22 Wisdom save or be magically compelled to harm an ally within range of its movement, using its strongest attack.

Chains of Betrayal (Recharge 5 turns). Azurnith summons spectral chains of black-gold to bind up to 3 creatures within 30 ft, lasting 3 rounds. Each creature must make a DC 22 Strength save or be restrained. While restrained, they take 18 (4d8) psychic damage at the start of their turns.

Command the Oathbound. Azurnith chooses up to 2 creatures charmed or frightened by him within 60 ft. Those creatures may immediately move up to half their speed and make one weapon attack as a reaction, as though compelled by unseen strings.

Revenge of the Betrayed. Azurnith teleports up to 60 ft. to an unoccupied space adjacent to a creature that has broken a vow or deal (DM’s discretion, but you can treat this as anyone who made a promise in-game and failed) in the last 30 days. His next attack against that creature this turn has advantage.

Vow of Retribution. When a creature within 60 ft. of Azurnith attacks him after previously swearing alliance, truce, or peace, he can force that creature to make a DC 22 Charisma save. On a failure, the creature’s attack automatically misses, and it takes 27 (6d8) psychic damage as the broken vow burns inside its soul.

Oathkeeper’s Grasp. When a creature that has broken a vow or deal in the last 30 days is within 10 ft. of Azurnith and tries to leave his reach, he may attempt to bind them with spectral chains of parchment and ink. The creature must succeed on a DC 22 Strength save or be restrained until the start of its next turn.

Azurnith can take 3 legendary actions, choosing from the options below:

Vowsteal. Azurnith targets one creature within 30 ft. That creature must succeed on a DC 22 Charisma save or be silenced for 1 round.

Promise Shatter (Costs 2). Azurnith forces one creature within 60 ft. to relive a moment they broke faith. They must succeed on a DC 22 Wisdom save or take 27 (6d8) psychic damage and fall prone.

Oathstrike (Costs 3). Azurnith makes one Blade of the Oathbound attack against every creature of his choice within 60ft, that has broken a vow in the last 30 days.

Mythic Trait – The Broken Ascendant (Recharges after Long Rest). When reduced to 0 hit points, Azurnith instead shatters into radiant-black light, regains 300 hit points, and his form twists into a darker, more terrifying visage.

For the remainder of combat, his Aura of Oaths, and Keeper of Vows radius increases to double, and every attack that hits him causes the attacker to take up to 12 psychic damage if they have broken a vow in the last year, scaling per month.