Colossus

Energy Absorption. Whenever the colossus is subjected to thunder or lightning damage, it takes no damage and instead regains a number of hit points equal to the damage dealt.

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

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

Magic Weapons. The Colossus's attacks are magical.

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

Ultimate Weapon. The Colossus deals double damage to objects, structures, and ships.

 Ancient Vehicle. The Colossus was designed to be used as a vehicle for an ancient people with utter disdain for all of the multiverse's living things. Inside of the Colossus's head is a 30-foot, cubic room with an ancient machine of massive rotating concentric bronze rings with glowing azure liquid floating in mystical wire-like thin streams throughout them. A creature can step through an opening in these rings and, as an action, attempt to take control of the Colossus. If it does, it makes a DC 37 Intelligence(Persuasion) check, taking complete and total control over the Colossus on a success. A creature in control begins to float in the center of the rotating rings and cannot be forced to leave by any means. The creature's movements are perfectly mirrored by the Colossus, though any resistance met by the Colossus is met by the creature in control of it. The creature may use a bonus action to leave the rings, at which point the Colossus maintains a telepathic link with the creature and obeys their beck and call to the best of its abilities, though no longer able to use Mythic Actions until the creature is back in control. Creatures within the ring machine do not need food, water, or air, and do not age. In addition, they cannot take damage besides Psychic, and cannot die unless the Colossus does, in which case the creature immediately dies. When a creature gains control of the Colossus for the first time, it is granted the immense knowledge of the ancients. Its Intelligence score is immediately rounded up to the nearest increment of five (minimum of an increase of 1, meaning that a creature with an Intelligence score divisible by five has its score increased by five).

Multiattack. The Colossus makes four different attacks. Two uses of the Cranial Cannon count as one attack.

Turbojet Punch. Melee Weapon Attack: +19 to hit, reach 50 ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage.

Cranial Cannon. Ranged Weapon Attack: +19 to hit, reach 500 ft., one target. Hit: 63 (10d10 + 8) lightning damage.

Crush. Melee Weapon Attack: +19 to hit, reach 50 ft., one target. Hit: 190 (40d8 + 10) nonmagical force damage.

Force Beam. Ranged Weapon Attack: +19 to hit, reach 1000 ft., one target. Hit: 175 (30d10 + 10) force damage.

Rotary Cannon. Ranged Weapon Attack: +19 to hit, reach 1000 ft., one target. Hit: 118 (20d10 + 8) piercing damage.

Rocket Blast (Recharge 4, 5, 6). The Colossus launches a massive burst of fiery magic up to five hundred feet, which explodes in a 50-foot radius sphere that spreads around corners but not through solid objects. Each creature, ship, and structure in that area must make a DC 20 Dexterity saving throw, taking 250 (100d4) fire damage on a failed save, or half as much damage on a successful one.

Reactor Burst (Recharge 6). The Colossus causes its high-energy Confinement Reactor to explode with a concentrated blast that radiates out 500 feet in all directions. Each creature, ship, and structure in that area must make a DC 20 Constitution saving throw, taking 500 (90d10+5) radiant damage on a failed save, or half as much damage on a successful one. The Colossus takes a quarter of this damage, and any creature in control must make a DC 5 Constitution saving throw or it and the Colossus die immediately. If it does not die, the Colossus gains an additional two legendary actions every round, to a maximum of seven.

Refuel. The Colossus regains 3d10 hit points.

Bolster Weapons. Until the start of its next turn, the Colossus takes double damage from attacks, but deals double damage with all of its own.

Bolster Shields. Until the start of its next turn, the Colossus takes half damage from attacks, but deals only half damage with its own.

Bolster Thrusters. Until the start of its next turn, the Colossus takes 5d4 additional damage every time it takes damage and damage it deals is reduced by 5d4, but its flying and walking speeds are doubled.

Block. The Colossus makes a Crunch attack against a creature or ship that moves at least 15 feet away from it. On a hit, the creature's or ship's movement is reduced to 0 until the start of its next turn, and remains halved until the end of that turn. The Disengage action offers no protection from this effect.

The Colossus can take 3 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature's turn. The Colossus regains spent legendary actions at the start of its turn.

Dash. The Colossus moves up to 150 feet in any direction.

Decimate. The Colossus pulls a creature or ship within 500 feet as far as it chooses directly toward itself, then, if possible, may make a Crush attack.

Recharge. The Colossus gains 5d8 temporary hit points. Then, the Colossus rolls a d6. On a 4 or 5, the Rocket Blast attack recharges. On a 6, the Reactor Burst recharges. On a 1-3, the Colossus regains 5d4 hit points.

Force Beam (Costs 2 Actions). The Colossus makes one Force Beam attack.

Fire Blast (Costs 3 Actions). If it is charged, the Colossus makes one Rocket Blast attack

While a creature is in direct control of the Colossus, it can use the options below as legendary actions.

Cast a Spell (Costs 5 Actions). The Colossus casts any spell fifth level or below as an eighth-level spell. The spell must have a casting time of 1 Action or less. The Colossus's spell attack modifier is +17 and its Spell Save DC is 25. 

Unload (Costs 7 Actions). The Colossus unloads the Rotary Cannon's entire cache of ammunition. It makes twelve Rotary Cannon attacks against the same target. After using this actions, the Colossus can no longer use the Rotary Cannon attack or this legendary action until it uses an action to reload the Cannon.