Ancient Void Dragon

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

Crawling Eyes. The Void Dragon has eight eyes that seem to slide along its body to wherever they need to be, completely independent of each other. If these eyes are targeted, their AC is 25 instead, and the resistances listed are changed to vulnerabilities, but radiant becomes a resistance. If any one eye takes 30 damage, it bursts into smoke, and disappears.

Inner Soul. The Void Dragon rarely opens its mouth, only to breathe its fire. This is because the fire that burns inside it is its greatest weakness. Like its eyes, the AC of its mouth is 25, and resistances become vulnerabilities, and radiant becomes a resistance. However, fire damage now does quadruple damage instead of just double. If it takes damage in its mouth, it stutters and a creature's save vs its breath weapon is made at advantage. If it is hit with fire damage while about to use its breath weapon, it does not use its breath weapon, it loses all actions including legendary actions, and its movement speed becomes 0. Only creatures within the cone of its breath weapon or aligned with it can take advantage of this opportunity with a held action, or reaction if they have a special ability to do so. If they attack the inner soul with a melee attack, they must save against the Celestial Soul Breath or take the allotted damage, save halves. Naturally, the Void Dragon only opens its mouth to use its breath weapon. Once the breath weapon is out of its mouth, it is treated as normal fire except for the radiant damage it deals, and attacking it does no damage.

Frightful Presence. Upon first seeing the dragon, each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 21 Wisdom saving throw or become frightened for 1 minute. A 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, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Black Hole Siege Monster. The tarrasque deals double damage to objects and structures. Additionally, it can pass through inanimate objects if it wishes, absorbing them as it goes, but its attacks against creatures are made with disadvantage if it does so.

Stubborn Undeath. When this creature is brought to 0 hit points, it explodes in a 100 foot burst of necrotic energy. On a failed save, creatures in the radius take 10d10 necrotic damage, half on a success. Where it died, there is a 15 ft square of radiant fire, as left by its celestial soul breath, but this square cannot be put out. After 1 minute, the square reconstitutes itself as a soul beast, a manifestation of the dragon's creator's soul. In this case, it becomes a Bladed Soul Arachnid, and immediately begins chasing after its creator to kill them, killing all who get in its way.

Multiattack. The dragon can make three attacks: one with its tail and two with its hands
Claw. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 24 (4d6 + 10) necrotic damage.
Tail. Melee Weapon Attack: +15 to hit, reach 40 ft., one target. Hit: 28 (4d8 + 10) necrotic damage.

Celestial Soul Breath (Recharge 6). The dragon exhales white fire in a 90‐foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking (13d6) fire damage and (13d6) radiant damage on a failed save, or half as much damage on a successful one. The area affected by the attack also continues to burn, becoming difficult terrain and causing anyone who crosses through that area during their turn to take 1d6 fire damage and 1d6 radiant damage.

Skin of an Event Horizon. If a creature melee attacks a Void Dragon, it can use its reaction to absorb the attack, taking half damage from it and attempting to tear the offending weapon from its owner's grasp. The creature must make a DC 30 Strength save or have its weapon pulled into the void. The save can be made with Dexterity instead if the creature is better in that. Magical weapons make this save with advantage.

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.
Predict. The dragon makes a Wisdom (Perception) check against an opponent's passive Stealth (10 + Stealth modifier) (Unless, of course, the opponent is actively using Stealth). If it succeeds, it locks one eye on an enemy, never letting it leave its sight. While locked on, the dragon can use its reaction against that creature without spending a reaction. It can do this with up to seven creatures at once, or as many as it has eyes left minus one, but must make the perception checks one at a time each costing a legendary action. 
Tail Attack. The dragon makes a tail attack.
Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 100 feet of the dragon must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Additionally, any unprotected flame within 100 feet, even the remnants of its breath weapon, are snuffed out. Protected flame has a 50% chance of being snuffed out.