Adamantine Body. Critical hits against the Vonindod count as normal hits instead. The Vonindod's attacks are magical, and when the Vonindod hits an object, the attack is an automatic critical hit.

Chromatic Absorbtion. When the Vonindod is subjected to acid, cold, fire, lightning or poison damage, it takes no damage and instead regains an amount of hit points equal to the damage.

Crushing Steps. When the Vonindod moves, it ignores difficult terrain. If the Vonindod enters a space occupied by another creature, that creature can use its reaction to move out of the Vonindod's path.  A creature that does not move out of the Vonindod's path must succeed on a DC 27 Strength saving throw or take 33 (4d12+10) bludgeoning damage and be knocked prone. Until the Vonindod moves again, the creature also is restrained, crushed beneath the Vonindod's feet.
While restrained in this way, the creature (or another creature within 5 feet of it) can use its action to make a DC 27 Strength (Athletics) check. On a success, the creature relocates to an unoccupied space of its choice within 5 feet of the Vonindod and is no longer restrained.

Dragon Slayer. The Vonindod always knows the exact location of all dragons within 5 miles of it and has advantage on attack rolls against dragons.

Immovable Colossus. The Vonindod is immune to forced movement.

Immutable Form. The Vonindod is immune to any spell or effect that would alter its form.

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

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

Siege Monster. The Vonindod deals double damage to objects and structures.

Towering Terror. Each creature that starts its turn within 120 feet of the Vonindod and is  aware of it must succeed on a DC 27 Wisdom saving throw or become frightened until the start of its next turn. Dragons have disadvantage on the saving throw. If a creature's saving throw is successful, it is immune to the Vonindod's Towering Terror for the next 24 hours.

Multiattack. The Vonindod uses its greatsword twice, makes an attack with its Horned Helm and one with its Wyrmsdoom Harpoon.

Dragonsbane Greatsword. The Vonindod swings its massive greatsword, hitting all creatures either in a 30-foot cone or in a line 60 feet long and 5 feet wide. Each creature within the area must make a Dexterity saving throw, taking 38 (8d6 + 10) slashing damage on a failure or half as much on a success.
If a creature fails its saving throw by 5 or more, it is knocked prone.
Dragons have disadvantage on the saving throw and take an additional 14 (4d6) slashing damage on a failed save or half as much on a success.

Horned Helm. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 54 (8d10 + 10) piercing damage. If the attack is a critical hit, the creature is impaled by the horns. An impaled creature is  restrained, moves with the Vonindod and cannot regain hit points until it is freed. It can use its action to free itself by succeeding on a DC 27 Strength (Athletics) check.

Wyrmsdoom Harpoon. Ranged Weapon Attack: +19 to hit, range 600/2400 ft., one target. Hit: 46 (8d8 + 10) piercing damage plus  18 (4d8) force damage. The Vonindod can only use its harpoon against targets that are more than 60 feet away. On a hit, the target is grappled (escape DC 27) as enormous adamantine chains wrap around it. While grappled, the target is restrained and the Vonindod cannot make attacks with its harpoon.

Runic Smite (3/day). Runes on the Vonindod's greatsword light up. The Vonindod can choose one of the following effects, which is triggered the first time the Vonindod uses its greatsword on this turn:

• Fire: The sword deals an additional 14 (4d6) fire damage. Each creature that is knocked prone by the attack also is restrained by massive fiery shackles. Until it frees itself, it takes 14 (4d6) fire damage at the start of each of its turns. It can use its action to free itself by succeeding on a DC 27 Strength (Athletics) check.
• Ice: The sword deals an additional 18 (4d8) cold damage. The area covered by the sword's swing becomes difficult terrain for 1 minute as it is covered in sharp spikes of ice and a creature takes 5 (2d4) piercing damage and 4 (1d8) cold damage for every 5 feet it travels within the area.
• Storm: The sword deals an additional 21 (6d6) lightning damage. Each creature that is knocked prone by the attack also is thrown up to 60 feet away from the Vonindod and cannot use reactions until the start of its next turn.

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

Legend's End. When a creature the Vonindod can see fails a saving throw and uses its Legendary Resistance trait to succeed instead, the Vonindod can force the creature to reroll the saving throw.

Anti-Magic Missile. Ranged Weapon Attack: +19 to hit, range 150/600 ft., one target. Hit: 20 (4d4+4) force damage. When the Vonindod sees a creature casting a spell within range, it can make an attack with this missle against it. If the attack hits, the target must succeed on a DC 27 saving throw using it spellcasting ability or its spell fails.

The Vonindod 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 tarrasque regains spent legendary actions at the start of its turn.

Attack. The Vonindod makes an attack with its Horned Helm.

Force Pull. The Vonindod pulls a creature grappled by its Wyrmsdoom Harpoon up to 60 feet straight toward it. The grappled creature takes 18 (4d8) force damage and must make a DC 27 Constitution saving throw. On a failure, it is unable to cast spells until the end of its next turn.

Move. The Vonindod moves up to half its speed.

Greatsword (Costs 2 Actions). The Vonindod uses its greatsword.