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

Spellcasting. The Dominator is a 3rd-level spellcaster. Its spellcasting ability is strength (spell save DC 10, +6 to hit with spell attacks). The Dominator has following wizard spells prepared:
Cantrips (at will): poison spray, fire bolt, light 
1st level (4 slots): thunderwave, burning hands
2nd level (3 slots): invisibility, blur
3rd level (4 slots): dispel magic, fireball, lightning bolt

Metal and Magic (Mythic Trait, 1/day). When the Dominator drops to 0 HP, it doesn't die or fall unconscious. Instead, it heals 20d20+100. The Dominator's AC increases to 30. A small crack should appear on its back, which can be further opened with earthquake. Once it is open, a creature may teleport or climb into the crack. This gives them access to the Dominator's inner workings. They can from here choose to attack the heart, which has an AC of 25, and 200 HP. Killing the heart petrifies the colossus, but after 1 hour it regains all lost hit points and the battle starts over. While it is petrified, the attackers may create a demiplane to imprison it, or cast imprisonment. They may also use wish, although this is not as likely to work.

Quake. The Dominator pummels the ground, creating a shockwave in a 50 ft. cylinder. Any creatures caught in the cylinder take 5d6 bludgeoning damage, and is knocked prone. The AOE then becomes difficult terrain.

Greatsword. Melee Weapon Attack: +10 to hit, reach 20 ft., 1 target. Hit: 32 (4d8 + 10) slashing damage. 

Light Beam. Ranged Weapon Attack: +10 to hit, range 20/360 ft., 1 target. Hit: 96 (10d10 + 6) radiant damage.

Frightful Presence. Each creature of the Dominator's choice within 120 feet of it and aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Dominator is within line of sight, 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 Dominator's Frightful Presence for the next 24 hours.

Punch. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 28 (4d8 + 10) slashing damage.

The Dominator 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 Dominattor regains spent legendary actions at the start of its turn.

Attack. The Dominator makes an attack action.

Move. The Dominator moves 50 ft. in a straight line.

Cantrip. The Dominator casts a cantrip.

Prepare. The Dominator prepares itself for the next attack, and gains +1 AC for the remainder of the round.

Earthquake. The Dominator uses its quake action.