Chthonian

Tunneler. The Chthonian can burrow through solid rock at half its burrow speed and leaves a 10-foot-diameter tunnel in its wake.

Innate Spellcasting (Psionics). The Chthonian’s innate spellcasting ability is Intelligence (spell save DC 16). It can innately cast the following spells, requiring no components:

• At will: mending (targeting stone/earth structures only)
• 3/day each: earth tremor, shatter
• 1/day: earthquake (centered on itself; the Chthonian is immune to the effects of its own earthquake spell)

Psychic Resonance. A creature that starts its turn within 30 feet of the Chthonian and is aware of it must succeed on a DC 16 Wisdom saving throw or become frightened until the start of its next turn. While frightened this way, a creature's speed is halved.

Multiattack. The Chthonian makes three attacks: one with its Bite and two with its Tentacles.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 19 (3d8 + 6) piercing damage plus 7 (2d6) acid damage.

Tentacles. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 13 (2d6 + 6) bludgeoning damage. If the target is a Large or smaller creature, it is grappled (escape DC 16). The Chthonian has twelve tentacles, but can only grapple up to four creatures at a time.

Seismic Pulse (Recharge 5–6). The Chthonian slams its massive body against the surrounding earth, sending out a violent shockwave. Each creature on the ground within 60 feet of the Chthonian must make a DC 18 Dexterity saving throw. On a failed save, a creature takes 36 (8d8) bludgeoning damage and is knocked prone. On a successful save, a creature takes half as much damage and isn't knocked prone. Structures in this area take double damage.