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

Mystic Reflection. Any time it is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, it is unaffected. On a 6, it is unaffected, and the effect is reflected back at the caster as though it originated from it, turning the caster into the target.

Antithesis. The creature ignores resistance to any damage it deals, but not immunities. 

Ultimate End. When the void dragon is killed it explodes in a swath of celestial destruction. Each creature and object within 1 mile of the dragon take 55 (10d10) + 80 bludgeoning damage, 55 (10d10) + 80 force damage, and 55 (10d10) + 80 psychic damage. Each damage type can be reduced by half with a successful DC 25 Constitution saving throw. Additionally, a creature that fails two or three of the saving throws is affected by a plane shift spell and sent to a random plane. If it is sent to the plane it currently occupies, it appears 5d100 miles away in a random direction.

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

Bite. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 20 (2d10 + 9) piercing damage plus 14 (4d6) cold damage.

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage plus 7 (2d6) cold damage.

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage.

Aura of Madness. Each creature of the dragon’s choice that is within 120 feet of the dragon and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature repeats the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature fails the saving throw by 5 or more it is driven insane. An insane creature is frightened permanently, and behaves as if affected by confusion while it is frightened in this way. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the dragon’s Aura of Madness for the next 24 hours.

Breath Weapons (Recharge 4-6). The dragon uses one of the following breath weapons.

     Gravitic Breath. The dragon exhales a 90-foot cube of powerful localized gravity, originating from the dragon. Falling damage in the area increases to 1d10 per 10 feet fallen. When a creature starts its turn within the area or enters it for the first time in a turn, including when the dragon creates the field, it must make a DC 24 Strength saving throw. On a failure, the creature is restrained. On a success, the creature’s speed is halved as long as it remains in the field. A restrained creature repeats the saving throw at the end of its turn. The field persists until the dragon’s breath recharges, and it can’t use gravitic breath twice consecutively.

     Stellar Flare Breath. The dragon exhales star fire in a 90-foot cone. Each creature in that area must make a DC 25 Constitution saving throw, taking (13d6) fire damage and  (13d6) radiant damage on a failed save, or half as much damage on a successful one. The area affected by the attack also continues to burn, becoming difficult terrain and causing anyone who crosses through that area during their turn to take 1d6 fire damage and 1d6 radiant damage Per 5ft walked. 

Teleport. The dragon magically teleports to any open space within 100 feet.

Void Twist. When the dragon is hit by a ranged attack, it can create a small rift in space to increase its AC by 5 against that attack. If the attack misses because of this increase, the dragon can choose a creature within 30 feet to become the new target for that attack. Use the original attack roll to determine if the attack hits the new target.

Skin of an Event Horizon. If a creature melee attacks a Void Dragon, it can use its reaction to absorb the attack, taking half damage from it and attempting to tear the offending weapon from its owner's grasp. The creature must make a DC 30 (whatever stat was used to attack) save or have its weapon pulled into the void. Magical weapons make this save with advantage.

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. 

Predict. The dragon makes a Wisdom (Perception) check against an opponent's Stealth. If it succeeds, it knows the locations of the creatures phsycicly, never letting it leave its sight. While locked on, the dragon can use its reaction against that creature without spending a reaction. It can do this with up to seven creatures at once. 

Tail Attack. The dragon makes a tail attack. 

Void Slip (Costs 2 Actions). The dragon twists the fabric of space. Each creature within 50 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 13 (2d6 + 6) bludgeoning damage and be knocked prone. The dragon can then teleport to an unoccupied space within 40 feet.