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

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

Adamantine Resistance. The guardian has advantage on saving throws against spells and other magical effects. It is also immune to critical hits.

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

Towering Terror. Any enemy that starts its turn within 30 feet of the guardian must succeed on a DC 20 Wisdom saving throw or be frightened until the start of the enemy’s next turn. If the enemy’s saving throw is successful, it is immune to this colossus’s Towering Terror for the next 24 hours.

Multiattack. The guardian makes one hammerfist attack and one axehand attack, or three eldritch cannon attacks.

Hammerfist. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) bludgeoning damage. If the target is a creature, it must succeed on a DC: 20 Strength saving throw or be pushed up to 20 feet in any direction the guardian chooses.

Axehand. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) slashing damage, plus 11 (2d10) slashing damage if the target is prone. If the target is adjacent to another creature, that creature also takes the same amount of damage.

Eldritch Cannon. Ranged Weapon Attack: +15 to hit, range 300 ft., one target. Hit: 22 (4d10) force damage, and if the target is a creature, it must succeed on a DC: 20 Strength saving throw or is knocked prone.

Exhaust Vent (Recharge 5-6). The guardian opens vents in its hull to excrete a 20-foot-radius sphere of poisonous, yellow-green fog around it. The fog spreads around corners. It lasts until the end of the guardian's next turn. Its area is heavily obscured.

When a creature enters the area for the first time on a turn or starts its turn there, that creature must make a DC: 20 Constitution saving throw. The creature takes 5d8 poison damage on a failed save, or half as much damage on a successful one. Creatures are affected even if they hold their breath or don't need to breathe.

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

Eldritch Cannon. The guardian makes one eldritch cannon attack.

Thrusters (Costs 2 Actions). The guardian moves up to its movement without invoking opportunity attacks.

Stomp (Costs 3 Actions). The guardian stomps one of its feet at a point on the ground within 10 feet of it. Any creature in a 10-foot-radius, 10-foot-high cylinder centered on this point must succeed on a DC: 20 Dexterity saving throw or take 33 (6d10) bludgeoning damage and fall prone. Until the guardian uses its Stomp again or moves, the creature is restrained. While restrained in this way, the creature (or another creature within 5 feet of it) can use its action to make a DC: 20 Strength check. On a success, the creature relocates to an unoccupied space of its choice within 5 feet of the guardian and is no longer restrained.

Structures, as well as nonmagical objects that are neither being worn nor carried, take the same amount of damage if they are in the cylinder (no save).