Chthon, high earth elemental

Adamantite body. The physical form of the Chthons are made with the strongest and hardest minerals and gemstones known, which not only grants them an AC of 25, but also provides a damage reduction of 1d10 against all physical attacks. Of course that also applies to their attacks that are considered magic to overcome resistances and immunities.

Reflective Carapace. Any time the elemental is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the elemental is unaffected. On a 6, the elemental is unaffected, and the effect is reflected back at the caster as though it originated from the elemental, turning the caster into the target.

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

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

Earth Glide. The elemental can burrow through nonmagical, unworked earth and stone. While doing so, the elemental doesn't disturb the material it moves through.

Energy liberation. The body of a Chthon holds with difficulty an immense amount of spiritual energy. When the elementals are damaged beyond a point, they lose one of their shells and part of their power surges with a shock wave. Every time a high elemental loses 100 Hp, an explosion happens in a 75 feet radius forcing a Dexterity saving throw with a 21 DC that deals 4d10 of force damage on a fail and half of it on a success. When the Chthon dies it releases a final shockwave that casts an earthquake spell.

Multiattack. The elemental makes two attacks. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 30 (4d12 + 6) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until this grapple ends, the target is restrained, and the elemental can’t bite another target.

Claw. Melee Weapon Attack: +13 to hit, reach 15ft., one target. Hit: 22 (4d8 + 6) slashing damage.

Horns. Melee Weapon Attack: +13 to hit, reach 10ft., one target. Hit: 26 (4d10 + 6) piercing damage.

Tail. Melee Weapon Attack: +13 to hit, reach 20ft., one target. Hit: 18 (4d6 + 6) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be knocked prone.

Swallow. The elemental 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 56 (16d6) piercing damage at the start of each of the elemental`s turns.

If the tarrasque takes 60 damage or more on a single turn from a creature inside it, the tarrasque 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 the tarrasque. If the tarrasque dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

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

Attack. The Chthon makes one claw attack or tail attack.

Subterranean charge. The elemental can use its burrow movement speed and make a horn attack.

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