Death Collapse. If Tharizdun dies, it starts to collapse. While it can no longer take any actions, its Living Black Hole  trait remains active. Its pulling distance increases by 30 feet on each subsequent turn until Tharizdun collapses, as Tharizdun loses control over its gravitational acceleration. All creatures and objects that enter Tharizdun's space take 135 (30d8) force damage, have their age reduced by 1d10 years and are instantly transported to the Astral Plane.

In addition, at the start of each of Tharizdun'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 Tharizdun collapses.

After 2d4 turns have passed, Tharizdun finally collapses on its next turn, resulting in a massive shockwave rippling through the entire plane. When this happens, each creature within 1 mile of Tharizdun must make a DC 25 Constitution saving throw. On a failure, it takes 45 (10d8) force damage and 45 (10d8) bludgeoning damage. A creature within 500 feet or less of Tharizdun instead takes 90 (20d8) force damage and 90 (20d8) bludgeoning damage and is sent to the Astral Plane on a failure. Creatures take half as much damage on a successful saving throw. Any affected creature that is not sent to the Astral Plane instead is flung 10d10 x 10 feet away.

After 10d100 years have passed, Tharizdun is reborn somewhere within the Abyss and becomes active once again.

Dark God's Maw. A creature that enters Tharizdun's space has a 90% chance to be sent to the Abyss. If this happens, it takes 90 (20d8) force damage, its age is reduced by 1d10 years and it and travels through a wormhole to a random layer of the Abyss. 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 Tharizdun's space  are consumed and disintegrated instantly. Magical items are sent to the Abyss and have a 10% chance to have their properties altered in a way determined by the DM.

Entity of Entropy. Tharizdun 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, Tharizdun is immune to Divination magic and any Divination spell cast within or targeting a point within 1 mile of Tharizdun fails.

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

Innate Spellcasting. Tharizdun's spellcasting ability is Charisma (spell save DC 27). It can innately cast the following spells, requiring no components:

• at will: chain lightning, dispel magic, maddening darkness, plane shift (self only)

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

Living Black Hole. The area within 60 feet of Tharizdun is filled with blackness, cold and otherworldly slurping and splashing noises and works like a hunger of hadar spell. The area within 120  feet of Tharizdun counts as difficult terrain due to the intense gravity and is filled with magical darkness. Tharizdun'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 Tharizdun and is not grappled by its tentacles must succeed on a DC 25 Strength saving throw or be pulled 30 feet closer towards Tharizdun.

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

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

Multiattack. Tharizdun can use its Wail of Hunger and then makes four attacks, Alternatively, it uses its Gravity Surge and casts one of its spells.

Fangs of Darkness. Melee Weapon Attack, +19 to hit, reach 60 feet, one target. Hit: 36 (4d12+10) piercing damage plus 18 (4d8) necrotic damage. The target must make a DC 25 Wisdom saving throw or is blinded until cured by a lesser restoration or similar magic. A creature killed by that attack is immediately consumed and disintegrated by Tharizdun and can't be brought back to life by any means short of a wish spell.

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 is grappled (escape DC 20) by the tentacle. Until the grapple ends, the creature is restrained.

Wail of Hunger. Tharizdun emits deep and eerie wailing, moaning and grinding sounds. Each creature within 500 feet of Tharizdun must make a DC 25 Wisdom saving throw or be frightened by Tharizdun for 1 minute. Creatures that suffer from at least one madness induced by Tharizdun's effects or abilities have disadvantage on the saving throw as to them, it appears as if Tharizdun was talking to them.

Creatures frightened in that way also are deafened, as they hear nothing but Tharizdun'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 Tharizdun. They must use their movement and take the dash action on their turn to move towards Tharizdun.

A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.  If a creature's saving throw is successful, or the effect ends for it, it becomes immune to Tharizdun's Wail of Hunger for the next 24 hours.

Gravity Surge (Recharge 5–6).  Tharizdun emits powerful gravitty waves infused with otherworldly magic. Each creature within 500 feet of Tharizdun must make a DC 25 Constitution saving throw. On a failure, it takes 45 (10d8) force damage, gains a level of exhaustion and becomes corrupted. On asuccess, it takes only half as much 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 if 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 Tharizdun disintegrates a creature, it uses its reaction to emit a pulse of invisible, corrupting radiation. Each creature within 120 feet of Tharizdun must make a DC 25 Constitution saving throw, taking 22 (5d8) radiant damage on a failure or half as much on a success, while Tharizdun itself regains 22 (5d8) HP. In addition, creatures who fail the saving throw are poisoned by radiation 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 120 feet of Tharizdun casts a spell, Tharizdun can force it to make a DC 25 Wisdom saving throw. On a failure, the spell fails as Tharizdun consumes its energy and regains HP equal to 5 times the spell's level. 

Tharizdun 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. Tharizdun regains spent legendary actions at the start of its turn.

Attack. Tharizdun makes an attack with its Fangs of Darkness or Void Tentacles.

Move. Tharizdun moves up to its speed.

Magic of the Dark God (Costs 2 Actions). Tharizdun casts one of its spells.

Empower Singularity. (Costs 2 Actions)  All creatures within 1 mile of Tharizdun must make a DC 25 Strength saving throw or are pulled 30 feet closer towards it.

Consume Life Energy. (Costs 2 Actions). Tharizdun 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 Tharizdun and can't be brought back to life by any means short of a wish spell.

Whispers from another World (Costs 3 Actions). All creatures within 120 feet of Tharizdun start hearing weird, incomprehensible whispers in their minds. Each affected creature must make a DC 25 Intelligence saving throw. On a failure, it takes 36 (8d8) psychic damage, gains a long-term madness and suffers from the effects of a feeblemind spell until the end of its next turn. On a success, it takes half as much damage and suffers no additional effecrs.