Walking Colossus

Crumbling Destruction. When the colossus drops to 0 hit points and dies, it crumbles and is destroyed. Any creature on the ground within 30 feet of the crumbling colossus must make a DC 22 Dexterity saving throw, taking 44 (8d10) bludgeoning damage on a failed save, or half as much damage on a successful one.

Colossus. The colossus is considered larger than gargantuan and automatically succeeds on any shove or grapple check it makes against creatures smaller than it. The colossus can move through the space of huge or smaller creatures, and such creatures can move through the space of the colossus. Whenever a creature enters the space of the colossus, or the colossus enters the space of another creature, it can make one slam attack (requires no action).

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

Sentinal's Might (Recharges after a Short or Long Rest). If the colossus would be reduced to 0 hit points, its current hit point total instead resets to 350 hit points, and it regains any expended uses of Legendary Resistance. Additionally, the colossus can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 62,000 XP (124,000 XP total) for defeating the colossus after its Sentinel's Might activates.

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' weapon attacks are magical.

Multiattack. The colossus makes two slam attacks.

Slam. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 36 (4d12 + 10) bludgeoning damage.

Hurled Stone. Ranged Weapon Attack: +16 to hit, range 200/800 ft., one target. Hit: 43 (6d10 + 10) bludgeoning damage.

Kick. The colossus targets one creature it can see within 10 feet that is huge or smaller, the target must succeed a DC 15 Dexterity saving throw or take 28 (4d8 + 10) bludgeoning damage and get launched 150ft back and knocked prone. If the target hits a solid object as a result, they take an additional 18 (4d8).

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

Attack. The colossus makes one slam attack.

Fortify (Costs 3 Actions). The colossus prepares itself for an oncoming attack. Until the start of its next turn, the next time the colossus takes damage from a single source, it can reduce the total damage by half, potentially causing it to fall under the damage threshold.

If the colossus' Sentinel's Might trait has activated in the last hour, it can use the options below as legendary actions.

Kick. The colossus makes one Kick attack.

Shattering Stomp (Costs 3 Actions). The colossus unleashes an earth-quaking stomp. Each creature on the ground in a 100-foot-radius sphere centered on the colossus is knocked prone and must make a DC 25 Constitution saving throw or take 26 (4d12) bludgeoning damage, taking half as much damage on a success.

If the colossus uses this ability three times at any point, the effected area becomes difficult terrain and 1d6 fissures open up in locations chosen by the DM. The fissures are 3d4 x 10 feet deep and 10 feet wide, any creature on top of a fissure when it opens drops into it and takes falling damage, all structures take 50 points of damage at the start of the initiative until it drops to 0 hit points and is destroyed.