Concord of The Realms: Ashen Archon of the Falling Stars

Aura of Silence. When a creature starts its turn within 30 feet of the Ashen Archon, the Archon can force that creature to make a DC 21 Wisdom saving throw if the rider can see it. On a successful save, the creature is immune to this aura for the next 24 hours. On a failed save, the creature can't speak and is deafened until the start of its next turn.

Mount. If the Ashen Archon isn't mounted, it can use a bonus action to magically teleport onto the creature serving as its mount, provided the Ashen Archon and its mount are on the same plane of existence. When it teleports, the Ashen Archon appears astride the mount along with any equipment it is wearing or carrying.

While mounted and not incapacitated, the Ashen Archon can't be surprised, and both it and its mount have Advantage on Dexterity saving throws.

Radiant Rebirth (Recharges after a Long Rest). If the Archon is reduced to 0 hit points, it regains 50 hit points and springs back to its feet with a burst of radiance. Each creature of the archon's choice within 30 feet of it must succeed on a DC 21 Constitution saving throw, or the creature takes 18 (4d8) radiant damage and is blinded until the start of the Archon's turn.

Magic Resistance. The Archon has Advantage on saving throws against spells and other magical effects.

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

Multiattack. The Ashen Archon makes two attacks with its ashen blade or bolt of ash in any combination and uses Spellcasting. 

Ashen Blade. Melee Attack Roll: +13 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 8) slashing damage plus 17 (3d10) radiant damage.

Bolt of Ash. Ranged Attack Roll: +13 to hit, range 120 ft., one creature. Hit: 30 (4d10+8) necrotic damage, and the target can't regain hit points until the start of the ashen rider's next turn. 

Spellcasting. The Ashen Archon's spellcasting ability is Wisdom (spell save DC 21,+13 spell attack). The Archon can innately cast the following spells, requiring no material components: 

At Will: Command, Compelled Duel, Spare the Dying

3/Day each: Guiding Bolt (5th level version), Crusader's Mantle, Daylight

2/Day each: Banishment, Fireball (6th level version), Spirit Guardians, 

1/Day each: Blade Barrier

Parry. The Ashen Archon adds 5 to its AC against one attack roll that would hit provided it can see the attacker. 

Legendary Action Uses: 3. Immediately after another creature's turn, the Ashen Archon can expend a use to take one of the following actions. The Archon regains all expended uses at the start of each of its turns.

Attack. The Ashen Archon makes an attack using its ashen blade or bolt of ash.

Coordinated Assault (Costs 2 Actions). The Ashen Archon makes an attack using its ashen blade or bolt of ash, and then its mount can use its reaction to make a melee  attack.

Reduce to Ash (Costs 3 Actions). The Ashen Archon targets a creature it can see within 60 feet of it. The target must succeed on a DC 21 Constitution saving throw, or it takes 27 (5d10) necrotic damage and its hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest. If the target's hit point maximum is reduced to 0, its body and everything it is wearing and carrying, except for magic items, are reduced to ash. A creature reduced to ash can't be revived by any means short of a Wish spell.