Death Collapse. If the Void dies, it starts to collapse. Its Living Black Hole  trait remains active, but its pulling distance increases by 30 feet on each subsequent turn until the Void collapses, as the Void loses control over its gravitational acceleration. All creatures and objects that enter the Void itself, or whatever remains of it, take 90 (20d8) force damage and are instantly transported to the Astral Plane.

In addition, at the start of each of the Void's turns, all creatures within 500 feet of it  must make a DC 25 constitution saving throw, taking 9 (2d8) radiant damage on a failure or half as much on a success. This radiant damage increases by 9 (2d8) on each subsequent turn until the Void collapses.

After 2d4 turns have passed, the Void finally collapses on its next turn, resulting in a massive shockwave rippling through the entire plane. When this happens, all creatures within 1 mile of the Void must make a DC 25 constitution saving throw. On a failure, they take 90 (20d8) force damage and 90 (20d8) bludgeoning damage and are sent to the Astral Plane, on a success they take half as much damage and are flung 8d8 x 10 feet away.

Killing the Void might result in further consequences determined by the DM.

Event Horizon. A creature that enters the Void itself, has a 90% chance to be sent to a different plane of existence. If this happens, it takes 90 (20d8) force damage and travels through a wormhole to another, randomly determined plane. Otherwise, the creature is disintegrated immediately and can't be brought back to life by any means short of a wish spell.

Nonmagical objects entering the Void are consumed and disintegrated instantly. Magical items are thrown into the Astral Plane and have a 10% chance to have their properties altered.

Immutable Existence. The Void is immune to any spell or effect that would alter its form or send it to another plane of existence.

Legendary Resistance (5/Day). If the Void fails a saving throw, it can choose to succeed instead.

Living Black Hole. The Void, after all, is nothing else than a living  - and hungering - Black Hole. The area within 60 feet of the Void is filled with a perpetual hunger of hadar spell and the area within 120  feet of the Void counts as difficult terrain due to the intense gravity. The Void's gravity in that area is strong enough to hold creatures airborne even if they lack a flying speed.

A creature that starts its turn within 1 mile of the void and is not grappled by its tentacles must succeed on a DC 25 strength saving throw or be pulled 30 feet closer towards the Void.

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

Radiant Absorbtion. Whenever the Void is subjected to radiant damage, it takes no damage and instead regains a number of hit points equal to the radiant damage dealt.

Space-Time Anomaly. The Void ignores difficult terrain, neither nonmagical nor magical effects can reduce its speed and it is immune to forced movement. It doesn't require food, drink or air and can move through solid objects, consuming and disintegrating them in the process.

In addition,  the Void is immune to Divination magic and any Divination spell cast within or targeting a point within 1 mile of the Void fails.

Multiattack. The Void  can use its Wail of Hunger. It then makes 4 Void Tentacle attacks.

Void Tentacle.  Melee weapon attack, +19 to hit, reach 300 feet, one target. Hit: 28 (4d8+10) cold damage plus 9 (2d8) acid damage.

The target must make a DC 25 constitution saving throw. On a failure, it can't take reactions and is grappled (escape DC 20) by the tentacle. Until the grapple ends, the creature is restrained.

Wail of Hunger. The Void emits deep and eerie wailing, moaning and grinding sounds. All creatures within 500 feet of the Void must make a DC 25 wisdom saving throw or be frightened by the Void for 1 minute.

Creatures frightened in that way also are deafened, as they hear nothing but the Void's wail, and if they are concentrating, they need to make a DC 25 constitution saving throw at the start of each of their turns or drop concentration.

Creatures who fail the saving throw by 5 or more succumb to the wails and feel an urge to move towards the Void. They must use their movement and take the dash action on their turn to move towards the Void.

Frightened creatures can repeat the saving throw at the end of their turns, ending the effect on themselves on a success.  If a creature's saving throw is successful, or the effect ends for it, it becomes immune to the Void's wail for the next 24 hours.

Gravity Surge (recharge 5, 6).  All creatures within 500 feet of the Void must succeed on a DC 25 constitution saving throw or take 45 (10d8) force damage, take a level of exhaustion and become corrupted. Creatures who succeed on the saving throw take only half damage without any additional effects.

A corrupted creature's flesh twists in alien ways, as powerful gravitational forces pull on it. The creature starts losing memories of its past, has disadvantage on attack rolls and f it tries to cast a spell, it must first succeed on a DC 15 intelligence check, or the spell fails and is wasted. The corrupted creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Radiation Jets. When the Void disintegrates a creature, it uses its reaction to emit a pulse of radiation. All creatures within 120 feet of the Void must make a constitution saving throw, taking 22 (5d8) radiant damage on a failure or half as much on a success, while the Void itself regains 22 (5d8) HP. In addition, creatures who fail the saving throw are poisoned until the end of their next turn and shed dim light within a 5-feet radius, thus they can't benefit from being invisible.

Absorb Energy. When a creature within 500 feet of the Void casts a spell, the Void can force it to make a DC 25 wisdom saving throw. On a failure, the spell fails as the Void consumes its energy. The Void regains HP equal to 5 times the spell's level. 

The Void can take 5 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 Void regains spent legendary actions at the start of its turn.

Attack. The Void makes an attack with one of its Void Tentacles.

Move. The Void moves up to its speed.

Empower Singularity. (costs 2 actions)  All creatures within 1 mile of the Void must make a DC 25 strength saving throw or are pulled 30 feet closer towards it.

Absorb Life Energy. (costs 2 actions). The Void absorbs the life energy of one creature it is grappling. The creature must make a DC 25 constitution saving throw. On a failure, it gains a level of exhaustion and takes 45 (10d8) necrotic damage. On a success, it takes half as much damage without any additional effects. A creature killed by that action is immediately consumed and disintegrated by the Void and can't be brought back to life by any means short of a wish.

Whispers from another World (costs 3 actions). All creatures within 120 feet of the Void start hearing weird, incomprehensible whispers in their minds. They need to make a DC 25 intelligence saving throw. On a failure, they take 36 (8d8) psychic damage, gain an infinite madness, and suffer from the effects of a feeblemind spell until the end of their next turn. On a success, they take half as much damage and suffer no additional effecrs.