Engine Against The Order (Recharges after a short or long rest). If the dragon would be reduced to 0 hit points, its current hit point total instead resets to 500 hit points, it recharges its Breath Weapon, and it regains any expended uses of Legendary Resistance. Additionally, the dragon can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 90,000 XP (180,000 XP total) for defeating Abber, The Engine of Creation after its Engine Against The Order activates.

Legendary Resistance (4/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Creature Sense. The elder brain is aware of the presence of creatures within 5 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each one’s Intelligence score, but can’t sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner.

Innate Spellcasting (Psionics). The elder brain’s innate spellcasting ability is Intelligence (spell save DC 26, +18 to hit with spell attacks). It can innately cast the following spells, requiring no components:

At will: detect thoughts, telekinesis eldritch blast (3 blasts)

2/day each: dominate monster, hold monster, plane shift

Magical Armory. Abber, The Engine of Creation wears a ring of spell turning, and has 5 of the teeth of dahlver-nar implanted in its mouth. At the end of each of the dragons long rests, it must choose which two teeth it is attuned to for the day (it is always attuned to the ring of spell turning). Any teeth it is not currently attuned to provide no effect. The teeth it can choose from are;
The Mill Road Murders (massive halfling canine)
When you damage a target that hasn’t taken a turn in this combat, the target takes an extra 3d10 slashing damage from ghostly blades.
Ashes of Ages and Eternal Fire (jade humanoid bicuspid)
You can use the tooth to cast counterspell at 9th level. Once you use this property, it can’t be used again until the next dawn. Whenever you finish a long rest, if you haven’t used the tooth to counter a spell since your last long rest, your hit point maximum is reduced by 2d10. If this reduces your hit point maximum to 0, you die.
Incendax's Tooth (Ruby-veined dragon fang)
You gain immunity to fire damage, and as an action, you can exhale fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 26d6 fire damage on a failed save, or half as much damage on a successful one. After using the breath weapon, you gain 2 levels of exhaustion.
The Claws of Dragotha (broken translucent fang)
You can use the tooth to cast the create undead spell. Once this property is used, it can’t be used again until the next dawn. Each time you create an undead creature using the tooth, a skeleton, zombie, or ghoul also appears at a random location within 5 miles of you, searching for the living to kill. A humanoid killed by these undead rises as the same type of undead at the next midnight.
The Golden Age of Dhakaan (massive golden goblin bicuspid)
When you are hit by an attack and an ally is within 5 feet of you, you can use your reaction to cause them to be hit instead. You can’t use this reaction again until you finish a short or long rest.

Telepathic Hub. The elder brain can use its telepathy to initiate and maintain telepathic conversations with up to 14 creatures at a time. The elder brain can let those creatures telepathically hear each other while connected in this way.

Siege Monster. The dragon deals triple damage to objects and structures.

Unusual Nature. The dragon doesn’t require air or sleep.

Multiattack. The dragon makes one Bite attack, two Claw attacks, and two Tentacle attacks. It can replace its Bite attack with any one action that isn't it's breath weapon, its mind blast, or a magic item effect OR it can make one Bite attack or Tentacle attack, and then use its mind blast, breath weapon, or magic item effect. 

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 9) piercing damage plus 11 (2d10) psychic damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 11 (1d6 + 9) slashing damage.

Tentacle. Melee Weapon Attack: +18 to hit, reach 15 ft., one creature. Hit: 12 (1d8 + 9) psychic damage. If the target is Huge or smaller, it is grappled (escape DC 22). The dragon can have up to four targets grappled at a time.

Mind Blast (Recharge 4–6). The elder brain magically emits psychic energy. Creatures of the elder brain’s choice within 60 feet of it must succeed on a DC 26 Intelligence saving throw or take 32 (5d10 + 18) psychic damage and become stunned for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Psychic Link. The elder brain targets one creature it can perceive with its Creature Sense trait and establishes a psychic link with that creature. Until the psychic link ends, the elder brain can perceive everything the target senses. The target becomes aware that something is linked to its mind at the end if its next short or long rest, and the elder brain can terminate the link at any time (no action required). The target can use an action on its turn to attempt to break the psychic link, doing so with a successful DC 24 Charisma saving throw. On a successful save, the target takes 10 (3d6) psychic damage. The psychic link also ends if the target and the elder brain are more than 5 miles apart, with no consequences to the target. The elder brain can form psychic links with up to 14 creatures at a time.

Sense Thoughts. The elder brain targets a creature with which it has a psychic link. The elder brain gains insight into the target’s reasoning, its emotional state, and thoughts that loom large in its mind (including things the target worries about, loves, or hates). The elder brain can also make a Charisma (Deception) check with advantage to deceive the target’s mind into thinking it believes one idea or feels a particular emotion. The target contests this attempt with a Wisdom (Insight) check. If the elder brain succeeds, the mind believes the deception for 1 hour or until evidence of the lie is presented to the target.

Tadpole Brine Breath (Recharge 5–6). The dragon exhales brine in a 120-foot line that is 15 feet wide. Each creature in that area must make a DC 23 Constitution saving throw, taking 55 (10d10) psychic damage on a failed save, or half as much damage on a successful one. On a success or failure, if the creature isn’t a Construct or an Undead, it becomes infested with illithid tadpoles.

While infested, the creature takes 16 (3d10) psychic damage at the start of each of its turns. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves. If the creature is targeted by magic that ends a curse or restores 40 hit points or more, the tadpoles infesting the creature are killed instantly, ending the effect on the creature.

If a creature is reduced to 0 hit points while infested, the creature is stable but remains unconscious for 6d12 hours. When the period of unconsciousness ends, the creature transforms into a horrifying, twisted version of it's former self. Humanoids are immune to this transformation, and instead die. 

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action 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.

Tentacle. The dragon makes one Tentacle attack.

Psychic Pulse. The elder brain targets a creature within 120 feet of it with which it has a psychic link. Enemies of the elder brain within 10 feet of that creature take 10 (3d6) psychic damage.

Sever Psychic Link. The elder brain targets a creature within 120 feet of it with which it has a psychic link. The elder brain ends the link, causing the creature to have disadvantage on all ability checks, attack rolls, and saving throws until the end of the creature’s next turn.

Shatter Concentration (Costs 2 Actions). The dragon targets a creature it is grappling. The target’s concentration on a spell it has cast or an ability it is maintaining ends, and the target takes 19 (3d12) psychic damage.

Inhale and Salivate (costs 3 actions). Roll (1d6). On a 5 or higher, the dragon regains it's breath weapon. 

If the dragons Engine Against The Order has activated within the last hour, it can use the following options in addition to its other legendary actions. It can also make up to 4 legendary actions now. 

Psychic Calamity (costs 3 actions). The dragon unleashes a torrent of psychic energy, sufficient to rip realms to pieces. Any creature that the dragon can see must succeed on a DC 26 intelligence saving throw, or suffer (27d6) psychic damage, and become paralyzed for one minute, or until they suffer slashing, piercing, or bludgeoning damage. A creature that is paralyzed by this effect is also frightened until the end of their next turn. If a creature with an intelligence score of 4 or lower suffers psychic damage from this ability, they die instantly, their brain liquifying in their skull. 

Mind Mastery. If it has recharged, the dragon can use its mind blast action. If it has not recharged, roll (1d6). On a 6 or higher, the dragon regains its use of Mind Blast, and may use it immediately.