Colossal. Sardior is immune to forced movement, does not provoke opportunity attacks and cannot make opportunity attacks. 

In addition, whenever Sardior makes a weapon attack, regardless of whether the attack hits or misses, each creature within 10 feet of the attack's target other than the target or Sardior himself must succeed on a DC 25 Dexterity saving throw or take half the attack's damage.

Discorporation. When Sardior dies, his body is destroyed but his essence travels back to his domain in the Ruby Palace, and he is unable to take physical form for a time.

Immutable Form. Sardior is immune to any spell or effect that would alter his form, except for his own Change Shape action.

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

Psionic Mastermind. Sardior can concentrate on two spells at the same time, and when he casts a spell that targets one creature, he can target two creatures with it instead. In addition, Sardior's thoughts and location can't be discerned by magic.

Ruby Shards. Sardior is always surrounded by floating ruby shards, which cause the area within 60 feet of him to be difficult terrain and resonate with psychic energy and heat. If Sardior takes psychic or fire damage, each creature within 60 feet of Sardior takes that damage instead; Sardior takes none of the damage.

 Spellcasting (Psionics). Sardior is a 20th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 27, +19 to hit with spell attacks). He can cast the following spells, requiring no components:

• Cantrips: control flames, fire bolt, mage hand (invisible), mind sliver, vicious mockery
• 1st level (at will): charm person, dissonant whispers, shield, silvery barbs
• 2nd level (at will): immovable object, scorching ray, Tasha's mind whip, vortex warp
• 3rd level (at will): counterspell, hypnotic pattern, pulse wave, slow
• 4th level (3 slots): confusion, gravity sinkhole, Raulothim's psychic lance, wall of fire
• 5th level (3 slots): dominate person, modify memory, synaptic static, telekinesis
• 6th level (3 slots): disintegrate, gravity fissure, Fizban's platinum shield, mental prison
• 7th level (3 slots): fire storm, reverse gravity, plane shift
• 8th level (2 slots): feeblemind, mind blank, dominate monster
• 9th level (2 slots): psychic scream, ravenous void, wish

Telepathic Awareness. Sardior can sense the presence and location of any creature within 300 feet of him that has an Intelligence of 3 or higher, regardless of interposing barriers.
In addition, if a creature communicates telepathically with Sardior, he can read the creature's thoughts. If he does so, he learns the creature's surface thoughts — what is most on its mind in that moment. He may also learn the creature's greatest desire, gain insight into its reasoning (if any), its emotional state, and something that looms large in its mind (such as something it worries over, loves, or hates).

Veil of Stars. While Sardior is in darkness, he has advantage on Dexterity (Stealth) checks, and when he takes the Hide action, Sardior can choose to become invisible until he attacks or casts a spell by blending in with the darkness and the starry night sky.

Multiattack. Sardior can use his Dazzling Presence. He then makes two attacks with his claws, one with his bite and one with his tail and uses Telekinetic Grip twice.

Bite. Melee Weapon Attack: +17 to hit, reach 40 ft., one target. Hit: 34 [rollable](4d12 + 8) piercing damage plus 18 (4d8) psychic damage plus 18 (4d8) fire damage and Sardior can grapple the target as part of the attack.

Claw. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 32 (4d10+8) slashing damage plus 18 (4d8) psychic damage plus 18 (4d8) fire damage.  If the target is a creature, it must succeed on a DC 25 Strength saving throw or be flung up to 3d6 × 10 feet away from Sardior, landing prone

Tail. Melee Weapon Attack: +17 to hit, reach 60 ft., one target. Hit: 36 (4d12 + 8) piercing damage plus18 (4d8) psychic damage plus 18 (4d8) fire damage and the target must make a DC 25 Strength saving throw or be knocked prone.

Telekinetic Grip. Sardior targets a creature or an object he can see within 300 feet of him. No one target can weigh more than 50,000 pounds, and objects can't be targeted if they're being worn or carried. The target must make a DC 27 Strength saving throw or becomes restrained in Sardior's telekinetic grip.

At the end of his turn, Sardior can move each creature or object he has in his telekinetic grip up to 60 feet in any direction, but not beyond 300 feet of himself. In addition, he can choose any number of creatures restrained by his telekinetic grip and deal 18 (4d8) force damage and 18 (4d8) fire damage to them.

At the end of each of its turns, a creature restrained by Sardior's telekinetic grip can repeat the saving throw, freeing itself on a success.

Dazzling Presence. Each creature of Sardior's choice that is within 300 feet of the dragon and aware of him must succeed on a DC 27 Wisdom saving throw or become frightened for 1 minute. While frightened in this way, a creature's movement speed is reduced to 0, as it stares in awe and fear at the Ruby Dragon and all the glittering gems around him. A creature can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a creature's saving throw is successful or the frightened condition ends for it, the creature is immune to being frightened by Sardior's Dazzling Presence for the next 24 hours.

Breath Weapon. Sardior uses one of the following breath weapons; he cannot use the same breath weapon on two consecutive turns:

• Searing Mindfire. Sardior breathes psychic energy imbued with the searing flame of the ruby within a 300-foot cone. Each creature within that area must make a DC 27 Intelligence saving throw, taking 54 (12d8) psychic damage and 54 (12d8) fire damage on a failure or half as much on a success. In addition, on a failure, a creature gains a level of exhaustion and is under the effect of a confusion spell until the end of its next turn.
• Ruby Storm. Shardior creates a shower of sharp, scorching hot ruby shards. Each creature of his choice within 300 feet of him must make a DC 27 Dexterity saving throw. On a failure, a creature takes 72 (16d8) piercing damage and 18 (4d8) fire damage and ruby shards are lodged in its skin; on a success a creature takes half as much damage without any other effect.
  While ruby shards are stuck to a creature's skin, the creature's movement speed is halved, it takes 18 (4d8) fire damage at the end of each of its turns and it cannot benefit from resistance to psychic damage. The shards remain for 1 minute or until a creature uses an action to remove them.
• Shroud of Night. Sardior breathes the very essence of Night itself. For the next minute, the area wiithin a 1-mile radius centered on Sardior becomes shrouded in silence as per the silence spell and in nonmagical darkness; only the light of twinkling stars in the sky breaches that darkness. Each creature of Sardior's choice within the area must succeed on a DC 27 Wisdom saving throw or fall asleep. A sleeping creature is unconscious and wakes up as soon as it takes any damage.

Quicken Spell (5/day). Sardior casts one of his spells with a casting time of 1 action.

Change Shape. Sardior magically polymorphs into a humanoid or beast that has a challenge rating no higher than his own, or back into his true form. He reverts to his true form if he dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (Sardior's choice).

In a new form, Sardior retains his alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this bonus action. His statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Sardior can take a total of three reactions per round, although only one per turn.

Divine Denial. When a creature within 300 feet of Sardior that he can see casts a spell or targets Sardior with an attack, he can use a reaction to force it to make a DC 27 Wisdom saving throw. On a failure, the spell or attack fails. Creatures immune to the charmed are immune to this effect. Clerics of neutral-aligned deities can use their Divine Intervention to prevent Sardior from using this feature for 24 hours.

Psionic Disarmament. When a creature within 300 feet of Sardior makes a weapon attack, he can use his reaction to force that creature to make a DC 27 Strength saving throw. On a failure, Sardior disarms the creature of the weapon it was making the attack with and flings it up to 60 feet away from it.

Sardior 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 dragon regains spent legendary actions at the start of its turn.

Hide. Sardior takes the Hide action.

Rushing Attack. Sardior moves up to his speed and makes an attack with its claws or its tail.

Cast a Spell (Costs 2 Actions). Sardior casts a spell.

Collapse Distance (Costs 2 Actions). Sardior unleashes a surge of psionic magic, teleporting itself to an unoccupied space within 120 feet of him. Each creature within 60 feet of the space Sardior left must succed on a DC 27 Strength saving throw or take 18 (4d8) force damage and 18 (4d8) psychic damage and be knocked prone.

Mass Telekinesis (Costs 3 Actions). Sardior uses Telekinetic Grip on each creature of his choice within 300 feet of him. When he does so, each creature that fails its saving throw takes 18 (4d8) force damage and 18 (4d8) fire damage.

Scintillating Ruby Ray (Costs 3  Actions, Recharge 5–6). Ranged Spell Attack, +19 to hit, range 120 feet, one target. Hit: 36 (8d8) radiant damage,  36 (8d8) psychic damage and 36 (8d8) fire damage and the target must make a DC 27 Constitution saving throw or be blinded until the end of its next turn.
Sardior creates a ray of scintillating starlight that can be reflected by mirrors, gems and other reflective surfaces, increasing the ray's range by 60 feet for each reflection. In addition, if the ray hits a creature with ruby shards attached to it, Sardior can repeat the attack roll against three new targets within 60 feet of that creature.