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

Life Stealer. Whenever the dragon reduces a creature‘s maximum hit points it gains just as much temporary hit points that lasts for 1 round.  If the dragon has taken radiant damage since it’s last turn it only gains a quarter as much temporary hit points as it normally would (any maximum hit points lost due to this dragon can only be reverted by a wish spell or greater restoration)

Soul Guard. If the dragon reduces a creature’s maximum hit points to 0 (and the dragon reduced at least 50 hit points of that maximum) the dragon gains immunity to all damage besides radiant (which the dragon will have resistance to for the duration) for one round. 

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

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 16 (1d10 + 10) piercing damage plus 21 (6d6) necrotic damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 17 (2d6 + 10) slashing damage.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

Draining Presence. Each creature that is within 120 feet of the dragon must succeed on a DC 21Constitution saving throw or have it’s maximum hit points reduced by 14 (4D6) or half as much on a successful save, if the save fails by 5 or more it becomes poisoned until the end of the void dragon’s next turn

Draining Breath (Recharge 6). The dragon exhales a wave of necrotic energy in a 90-foot cone. Each creature in that area must make a DC 24 Constitution saving throw, taking 70 (20d6) necrotic damage on a failed save, or half as much damage on a successful save, if the creature failed the save it’s maximum hit points are reduced by half the necrotic damage taken

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.

Heavy Drain (Costs 2 Actions). The dragon uses it’s draining presence