Catastrophic quaken

Living Earthquake: The Catastrophic Quaken ignores difficult terrain made of stone or earth. Creatures standing on the ground within 30 feet of the dragon have disadvantage on Dexterity saving throws.

Seismic Hide: Whenever a creature hits the Quaken with a melee attack while standing on the ground, that creature must succeed on a DC 22 Dexterity saving throw or fall prone as the earth violently cracks beneath them.

Tunnel Colossus: The Quaken can burrow through solid stone without reducing its speed, leaving behind a tunnel large enough for Huge creatures.

Titanic Presence: Enemies that begin their turn within 60 feet of the dragon must succeed on a DC 19 Wisdom saving throw or become frightened until the start of their next turn.

Multiattack: The Catastrophic Quaken makes three attacks: one with its bite and two with its slam attacks.

Bite: Melee Weapon Attack: +17 to hit, reach 15 ft., one target.
Hit: 31 (4d10 + 10) piercing damage plus 10 (3d6) thunder damage.

Slam: Melee Weapon Attack: +17 to hit, reach 20 ft., one target.
Hit: 28 (4d8 + 10) bludgeoning damage.
If the target is Large or smaller, it must succeed on a DC 22 Strength saving throw or be knocked prone.

Seismic Roar (Recharge 5–6): The Quaken releases a catastrophic sonic eruption. Each creature on the ground within 120 feet must make a DC 22 Constitution saving throw.

• Failure: 63 (14d8) thunder damage, knocked prone, and stunned until the end of its next turn.
• Success: Half damage and not stunned.

Nonmagical structures in the area take double damage.

Earthbreaker Dive (Recharge 4–6): The Quaken burrows underground and erupts from beneath a point it can sense within 80 feet. Creatures within a 30-foot radius must make a DC 22 Dexterity saving throw.

• Failure: 45 (10d8) bludgeoning damage, knocked prone, and buried in rubble (restrained).
• Success: Half damage and pushed 10 feet.

A restrained creature can use an action to make a DC 20 Strength check to escape.

Legendary Actions: The Catastrophic Quaken 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.

Detect: The dragon makes a Wisdom (Perception) check.

Tremor Step: The Quaken moves up to half its burrow speed without provoking opportunity attacks.

Tail Quake (Costs 2 Actions): The dragon slams its tail into the ground. Creatures within 15 feet must succeed on a DC 22 Dexterity saving throw or take 18 (4d8) bludgeoning damage and fall prone.

Cataclysm Pulse (Costs 3 Actions): The dragon emits a concentrated seismic wave in a 60-foot line that is 10 feet wide. Creatures in the line must make a DC 22 Constitution saving throw, taking 27 (6d8) thunder damage on a failed save, or half as much on a success.