Crugin

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

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

Siege Monster. Crugin deals double damage to objects and structures.

Storm Bringer. Crugin is surrounded overhead by a perpetual storm that spans out up to 3 miles unless Crugin is unconscious. The storm brings the occasional flash of lightning, rumble of thunder and rainfall, which extinguishes unprotected non-magical flames after a short time.

Multiattack. Crugin can use its Frightful Presence. It then makes four attacks: one with its bite, two with its claws, and one with its tentacle. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +15 to hit, reach 5 ft., one target. Hit: 28 (3d12 + 9) piercing damage. If the target is a creature, it is grappled (escape DC 23). Until this grapple ends, the target is restrained, and Crugin can't bite another target.

Claw. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 22 (3d8 + 9) slashing damage.

Tentacle. Melee Weapon Attack: +15 to hit, reach 30 ft., one target. Hit: 19 (3d6 + 9) bludgeoning damage, and the target is grappled (escape DC 23). Until this grapple ends, the target is restrained. Crugin has two tentacles, each of which can grapple one target.

Frightful Presence. Each creature of Crugin'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 Crugin 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 Crugins Frightful Presence for the next 24 hours.

Swallow. Crugin makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 42 (12d6) acid damage at the start of each of Crugin's turns.

If Crugin takes 40 damage or more on a single turn from a creature inside it, Crugin must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Crugin. If Crugin dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Primal Storm (1/Day). Crugin enhances the churning storm cloud overhead, centred on itself and spreading to a radius of 360 feet. Lightning flashes in the area, thunder booms, and strong winds roar. Each creature under the cloud (no more than 5,000 feet beneath the cloud) when it appears must make a DC 21 Constitution saving throw. On a failed save, a creature takes 7 (2d6) thunder damage and becomes deafened for 5 minutes.

If Crugin takes more than 50 points of damage during a round before the start of its next turn, it must make a DC 20 Constitution saving throw to maintain concentration on the storm. Every time Crugin takes damage after passing the damage threshold, it must make a regular Concentration check. The storm produces different effects on the start of Crugin's turns.

Round 2. Acidic rain falls from the cloud. Each creature and object other than Crugin under the cloud takes 7 (2d6) acid damage.

Round 3. Six bolts of lightning strike up to six creatures or objects of Crugin's choice beneath the cloud. A given creature or object can't be struck by more than one bolt. A struck creature must make a DC 21 Dexterity saving throw. The creature takes 35 (10d6) lightning damage on a failed save, or half as much damage on a successful one.

Round 4. Hailstones rain down from the cloud. Each creature other than Crugin under the cloud takes 7 (2d6) bludgeoning damage.

Round 5–10. Gusts and freezing rain assail the area under the cloud. The area becomes difficult terrain and is heavily obscured. Each creature other than Crugin takes 3 (1d6) cold damage. Ranged weapon attacks in the area are impossible. The wind and rain count as a severe distraction for the purposes of maintaining concentration on spells for creatures other than Crugin. Finally, gusts of strong wind (ranging from 20 to 50 miles per hour) automatically disperse fog, mists, and similar phenomena in the area, whether mundane or magical.

Crugin 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. Crugin regains spent legendary actions at the start of its turn.

Attack. Crugin makes one claw attack or tentacle attack (cannot grapple with tentacle).

Move. Crugin moves up to half its movement speed.

Chomp (Costs 2 Actions). Crugin makes one bite attack or uses its Swallow.