Otherworldly Nature. The dragon does not need to eat, sleep, or breath.

Void Awakening (Recharges after a Short or Long Rest). If the greatwyrm 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 greatwyrm can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 90,000 XP (180,000 XP total) for defeating the greatwyrm after its Void Awakening activates.

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

Collapsing Void. When the greatyrm is killed, the black hole leading to the Void Plane at its core begins to collapse and then explode. Each creature within 1 mile of the dying Void Dragon takes 55 (10d10) bludgeoning damage, 55 (10d10) necrotic damage, and 55 (10d10) psychic damage. This damage can be reduced by half with a DC 21 Dexterity saving throw, Constitution saving throw, and Wisdom saving throw respectively, and the effects of the Evasion feature are disregarded if a creature has this feature.

Any creature who fails 2 or more saving throws rolls a d100.  The creature is teleported to another random plane of existence.  Consult the table underneath Lair Actions to determine what plane a player is sent to.

Multiattack. The greatwyrm can use its Aura of Madness.  It then makes three attacks: one with its bite and two with its claws.

Aura of Madness. Each creature within 120 ft. of the greatwyrm must make a DC 26 Wisdom saving throw or be frightened by it for 1 minute. A creature can repeat this saving throw at the end of each of their turns.  A creature that fails by 5 or more is frightened of the greatwyrm permanently and rolls on the Long-Term Madness table, exhibiting the effects until the fear ends.  Only a lesser restoration spell or similar magic of 5th level or higher can remove these effects.   If a creature's saving throw is successful or the effect ends for it, the creature is immune to the greatwym's Aura of Madness for the next 24 hours.

Bite. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 19 (2d10 + 9) piercing damage plus 8 (2d8) necrotic damage and 8 (2d8) psychic damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 18 (2d8 + 9) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 19) and is restrained until this grapple ends. The greatwyrm can have only one creature grappled in this way at a time.

Tail. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 13 (2d8 + 9) bludgeoning damage.

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

Void Breath. The greatwyrm exhales purple and black necrotic energy in a 300-foot line that is 15 feet wide. Each creature in that area must make a DC 26 Dexterity saving throw. On a failed save, the creature takes 32 (7d12) necrotic damage or half as much on a successful one.  Creatures in the area must also make a DC 26 Intelligence saving throw or take 32 (7d12) psychic damage and become stunned until the end of their turn.

Gravitic Breath. The greatwyrm exhales a 300-foot cube of increased gravity originating from it. Any flying or levitating creature has its flying speed reduced to 0 and crashes to the ground, taking 6 (1d12) bludgeoning damage per 10 feet fallen and becomes prone. Any creature on the ground must succeed a DC 26 Strength saving throw or take 12 (3d8) force damage and have its movement reduced to 0.  The entire area is considered difficult terrain. The dragon cannot use this breath twice in a row and the field lasts until the end of the greatwyrm's next turn.

Spellcasting (Psionics). The greatwyrm casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 26):

At will: plane shift (Void Plane only)

1/day each: counterspell, forcecage, reverse gravity, time stop, hunger of hadar (as a 5th-level spell)

Void Slip. The dragon twists the fabric of reality. Each creature within 20 feet of the dragon must succeed on a DC 26 Dexterity saving throw or take 16 (2d8 + 8) force damage and be knocked prone. The dragon can then teleport to an unoccupied space within 60 feet.

The dragon 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.

Attack. The dragon makes one Claw or Tail attack.

Psionics (Costs 2 Actions). The dragon uses its Void Slip or Spellcasting.

Psychic Lance (Costs 3 Actions). The greatwyrm creates four lances of psychic energy. Each lance hits a creature of the greatwyrm’s choice it can see within 120 feet of it, dealing 14 (1d12 + 8) psychic damage to its target, then disappears.

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

Bite. The greatwyrm makes one Bite attack.

Inflict Madness (Costs 3 Actions). The greatwyrm unleashes its its truly horrifying visage.  Each creature in a 60-foot-radius sphere centered on the greatwyrm must succeed on a DC 26 Wisdom saving throw or become inflicted with a short-term madness.  A creature can repeat this saving throw at the end of each of its turns, ending the madness on a success.  A greater restoration or similar magic of 5th-level or higher can also remove these effects.