Void Greatwyrm

Void Flux (Recharges after a Short or Long Rest). If the greatwyrm would be reduced to 0 hitpoints, its current hit point total instead resets to 475 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the greatwyrm can not use the options in the "Mythic Actions" sectrion for 1 hour. Award a party an additional 135,000 XP (270,000 XP total) for defeating the greatwyrm after its Void Flux activates.

Innate Spellcasting. The greatwyrm's innate spellcasting ability is Charisma (spell save DC 26, +18 to hit). It can innately cast the following spells, requiring no material components:

At will: blur, dimension door, ray of enfeeblement

1/day each: dream, feeblemind, gate

Legendary Resistance (4/Day). If the greatwyrm fails a saving throw, it can choose to succeed instead.

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

Magic Weapons. The greatwyrm's weapon attacks are magical.

Alien Nature. The void greatwyrm doesn't require air, food or drink, and magic cannot put it to sleep.

Null Body. Whenever the greatwyrm is subjected to force damage, it takes no damage and instead regains a number of hit points equal to half the force damage dealt.

Multiattack. The dragon can use its Alien Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage plus 14 (4d6) force damage. If this damage reduces a creature to 0 hit points, its body becomes a pile of fine gray dust.

Claw. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 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 greatwyrm can have only once creature grappled this way at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Alien Presence. Each creature of the greatwyrm's choice that is within 300 feet of the dragon and aware of it must succeed on a DC 26 Wisdom saving throw or be afflicted with a random short-term madness effect for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the greatwyrm is within line of sight, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Alien Presence for the next 24 hours.

Breath Weapon (Recharge 5–6). The greatwyrm uses one of the following breath weapons:

Void Breath. The greatwyrm emits a destructive beam from its maws in a 300-foot line that is 30-feet wide. Each creature in that area must make a DC 27 Dexterity saving throw, taking 72 (11d12) force damage on a failed save, or half as much damage on a successful one. Non-magical objects within this area that are not being worn or carried by creatures are reduced to a pile of fine gray dust.

Suffocating Breath. The greatwyrm inhales all the air within a 300-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, or have the breath pulled from their lungs and begin to choke, following the rules for suffocation. For the next 1d6 rounds, the affected area is a vacuum and no air is present, preventing creatures from breathing within the area.

The greatwyrm can take 4 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.

Cast a spell. The greatwyrm casts a spell

Attack. The dragon makes one Claw or Tail attack.

Void Gaze (Costs 2 Actions). One creature of the dragon's choice within 60 feet of it must succeed on a DC 26 Charisma saving throw or become stunned until the end of their next turn.

Wing Attack (Costs 2 Actions). The greatwyrm beats its wings. Each creature within 30 feet of the greatwyrm must succeed on a DC 27 Dexterity saving throw or take 17 (2d6 + 10) and be knocked prone.

If the greatwyrm's Void Flux trait has activated in the last hour, it can use the options below as legendary actions.

Bite. The greatwyrm makes one Bite attack.

Void Surge (Costs 2 Actions). The greatwyrm's null energy proliferates rapidly. Each creature in a 60-foot radius sphere centered on the greatwyrm must succeed on a DC 27 Dexterity saving throw or take 22 (5d8) force damage. If this damage reduces a creature to 0 hit points, its body becomes a pile of fine gray dust.