Aspect of Null

Undying Wrath (recharges after a Short or Long Rest). If the aspect would be reduced to 0 hit points, its current hit point total instead resets to 500 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the aspect can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 155,00 XP (310,000 XP total) for defeating the Aspect of Null after its Undying Wrath activates.

Legendary Resistance (5/Day). If the aspect fails a saving throw, it can choose to succeed instead.

Multiattack. The aspect makes one Bite attack, one Claw attack, and one Tail attack.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 22 (4d10) necrotic damage. 

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. The aspect can only have one creature grappled this way at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 15., one target. Hit: 23 (2d12 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Reaping Breath (Recharge 5-6). The aspect inhales and draws in life in a 300-foot cone. Each creature in the area must make a DC 27 Constitution saving throw. On a failed save, the creature takes 71 (11d12) necrotic damage. On a successful save, the creature takes half as much damage. A creature reduced to 0 hit points from this attack dies and is unable to be restored to life without the use of a Wish Spell. Additionally, the aspect recovers half of the damage it inflicts with this attack.

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

Attack. The aspect makes one Claw or Tail attack.

Furious Bite (Costs 2 Actions). The aspect makes one Bite attack. If the attack hits a creature, the target must succeed on a DC 27 Wisdom saving throw or become frightened of the aspect until the end of its next turn.

If the aspect's Undying Wrath trait has been activate in the last hour, it can use the options below as legendary actions.

Death Shield (Costs 2 Actions). The aspect manifests an Avatar of Death for each hostile creature it is engaged with. These Avatars make a Reaping Scythe attack against each hostile creature and disappear immediately after. The aspect gains temporary hit points equal to the damage dealt until the start of its next turn.

Necrotic Flare (Costs 3 Actions). The aspect flares with necrotic energy. Each creature of the aspect's choice in a 60-foot-radius sphere centered on it must make a DC 27 Constitution saving throw. On a failed save, the creature takes 39 (6d12) Necrotic damage. On a successful save, the creature takes half as much damage.