Ancient Voidwoken Dragon

Magic Attacks. The Ancient Voidwoken Dragon's bite, claw, tail, and breath weapon attacks are magical.

Voidwoken Vulnerability. Any creature hit by the Ancient Voidwoken Dragon's bite, claw, or tail attacks becomes vulnerable to necrotic damage. This vulnerability stacks, and there is no limit to the number of stacks a creature can accumulate. This vulnerability also causes creatures to lose any immunity to necrotic damage while they have one or more stacks. The effect lasts until the creature finishes a long rest, at which point the vulnerability and the negated immunity end.

Void Leech. When the Ancient Voidwoken Dragon deals necrotic damage to a creature, it regains hit points equal to half the necrotic damage dealt.

Void Exhaustion. Any creature hit by the Ancient Voidwoken Dragon's bite, claw, or tail attacks must succeed on a DC 27 Constitution saving throw or gain one level of exhaustion.

Void Annihilation. If the Ancient Voidwoken Dragon comes into direct physical contact with a magical construct, the construct is instantly destroyed.

Phase Through. The dragon can move through other creatures and objects as if they were difficult terrain. If it ends its turn inside an object or creature, it takes 28 (8d6) force damage. Any creature whose space the dragon moves through while phasing takes 28 (8d6) necrotic damage.

Void Structure Demolition. The Ancient Voidwoken Dragon deals double damage to constructs and structures with its attacks and breath weapon.

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

Shadow Blend. While in dim light or darkness, the dragon can take the Hide action as a bonus action. While hidden in this way, the dragon is also invisible, and this invisibility cannot be seen through by truesight.

Multiattack. The dragon can use its Frightful Presence and one Void Mystic Action. 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: 23 (2d10 + 12) necrotic damage, and the target gains one stack of Voidwoken Vulnerability and must make a DC 27 Constitution saving throw or gain one level of exhaustion.

Claw. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 19 (2d6 + 12) necrotic damage, and the target gains one stack of Voidwoken Vulnerability and must make a DC 27 Constitution saving throw or gain one level of exhaustion.

Tail. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 21 (2d8 + 12) necrotic damage, and the target gains one stack of Voidwoken Vulnerability and must make a DC 27 Constitution saving throw or gain one level of exhaustion.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 27 Wisdom saving throw or become frightened for 1 minute. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Void Breath (Recharge 3-4). The dragon exhales a 90-foot cone of pure void energy. Each creature in that area must make a DC 27 Dexterity saving throw, taking 77 (22d6) necrotic damage and having its maximum hit points reduced by an amount equal to the necrotic damage taken on a failed save, or half as much damage and no reduction to maximum hit points on a successful one. Creatures that fail this saving throw also gain one stack of Voidwoken Vulnerability. The reduction to a creature's maximum hit points lasts until the creature finishes a long rest.

Invisibility. The dragon magically becomes invisible until it attacks or until its concentration is broken, up to 1 hour (as if concentrating on a spell). This invisibility cannot be seen through by truesight.

Void Blink: The dragon magically teleports up to 30 feet to an unoccupied space it can see.

Manifest Void Tendril: The dragon briefly manifests a small void tendril to lash out at a creature within 60 feet. The target must succeed on a DC 27 Strength saving throw or be restrained until the end of the dragon's next turn.

Void Retribution: When a creature within 30 feet of the dragon damages it, the dragon can unleash a burst of void energy as a reaction. The attacker must make a DC 27 Dexterity saving throw or take 21 (6d6) necrotic damage.

Reactive Phase: When the dragon is hit by an attack, it can use its reaction to phase partially out of reality, halving the damage it takes from that attack.

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.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 27 Dexterity saving throw or take 19 (2d6 + 12) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Phase Shift (Costs 2 Actions). The dragon magically phases up to its phase speed without provoking opportunity attacks.

Void Mystic Actions (1/Turn). The dragon can use one of the following mystic actions:

Void Lash. The dragon unleashes a tendril of void energy at one creature it can see within 120 feet. The target must succeed on a DC 27 Dexterity saving throw or take 31 (9d6) necrotic damage and be pulled up to 30 feet closer to the dragon.

Enervating Gaze. The dragon focuses its gaze on one creature it can see within 60 feet. The target must succeed on a DC 27 Constitution saving throw or have disadvantage on the next attack roll, saving throw, or ability check it makes for 1 minute.

Void Shroud. The dragon momentarily surrounds itself in swirling void energy. Until the end of its next turn, any creature that starts its turn within 15 feet of the dragon takes 14 (4d6) necrotic damage. This Shroud last for 1 minute.