Chasmic Wyrm

Earth Tremor. As a bonus Action, the Chasmic Wyrm can cause the ground  within a 60ft. radius to tremble. Each creature on the ground other than the Wyrm must succeed on a DC 18 saving throw or be knocked prone.

Gemstone Refract. When the Chasmic Wyrm takes fire or radiant damage, as a reaction it can refract the energy through its gemstones, causing all creatures within 30 ft. to make a DC 18 Dexterity saving throw. On a failed save, creatures take 4d8 fire damage, or half as much on a successful save.

Multiattack. The Chasmic  makes two attacks: one with its bite and one with its Tail.

Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 26 (3d10 + 10) piercing damage plus 9 (2d8) fire damage.

Tail Swipe. Melee Weapon Attack: +14 to hit, reach 15 ft., one creature. Hit: 23 (3d8 + 10) bludgeoning damage, and the target must make a DC 18 strength saving throw or be knocked prone.

Burrow Strike. (Recharge 4-6) The Wyrm burrows underground and emerges at a point within 60 feet, causing the ground to erupt. Each creature within 20 ft. of the point must make a DC 18 Dexterity saving throw, taking 22 (4d10) bludgeoning damage on a failed save, or half as much on a successful one.

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

Detect. The Chasmic Wyrm makes a Wisdom (perception) check.

Tail Attack. The Chasmic Wyrm makes a tail attack.

Earthquake (Costs 2 Actions). The Chasmic Wyrm causes an earthquake in a 40 ft. radius centered on itself. Each creature within 20 ft. of the point must make a DC 18 dexterity saving throw or be knocked prone and take 21  (6d6) bludgeoning damage. Structures and terrain in the area also take this damage.