Adunaphel

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

Magic Resistance. The specter has advantage on saving throws against spells and other magical effects.

Burning Soul (Recharges after a Short or Long Rest). If the Adunaphel is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 120 hit points. In addition, a wave of flames erupts from Adunaphel's body, each creature within 10 ft of Adunaphel must make a DC 16 Dexterity saving throw or takes 11 (2d10) fire damage or half as much on successful save and and all damage dealt by Adunaphel becomes fire damage  Additionally, the Adunaphel can now use the options in the “Mythic Actions” section. Award a party an additional 15000 XP (30000 XP total) for defeating the Adunaphel after it uses Burning Soul.

Multiattack. The specter uses Mournful Keening if available, then makes one Hooves attack and one Reaping Scythe attack or two Ice Needles attacks.

Hooves. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 16 (3d6 + 6) bludgeoning damage plus 28 (8d6) necrotic damage. If the target is a Medium or smaller creature, it must succeed on a DC 18 Strength saving throw or have the prone condition.

Mournful Keening (Recharge 6). The specter utters a keening wail, calling to those close to death. Each non-Undead creature within 120 feet of the specter must make a DC 16 Constitution saving throw. On a failure, the creature hears the wail and is marked for death.

creature marked for death can’t regain hit points, has disadvantage on death saving throws, and all attack rolls against it are made with advantage. This effect lasts for 1 minute or until the creature is targeted by a remove curse spell or similar magic.

Reaping Scythe. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 12 (1d12 + 6) slashing damage plus 28 (8d6) necrotic damage. If the target is a creature, it must succeed on a DC 15 Constitution saving throw or gain a level of exhaustion.

Ice Needle. Ranged Spell Attack: +9 to hit, reach 120 ft., one target. Hit: 18 (3d12) cold damage.

The Adunaphel 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 dragon regains spent legendary actions at the start of its turn.

Death eye. The Adunaphel looks with his murderous eyes at the person he can see. A creature he choose within 120 feet of him must make a DC 16 Constitution saving throw or take 17 (3d10) necrtoic damage on failed save or half as much on a successful one.

Ice Needle. The Adunaphel makes a Ice Needle attack.

Death Arrival (Costs 2 Actions). Specter teleports up to 100ft to a location he can see. All cratures within 10 ft of him when he teleports and from the place he arrives to must succed on a DC 16 Strength saving throw or take 15 (2d10 + 4) necrotic damage and be knocked prone.

If the Adunaphel's Burning Soul trait is active, it can use the options below as legendary actions.

Death Presence. Each creature of the Adunaphel's choice that is within 120 feet of the Adunaphel and aware of it must succeed on a DC 16 Wisdom saving throw or become frightened until the end of its next turn. 

Reaping Scythe (Cost 2 Actions). Adunaphel makes a Reaping Scythe Attack if it hits Adunaphel gain 10 temporary HP.