Xandaltheldrumorr, the Dune Dreadwyrm

Deep Breaths. Xandaltheldrumorr can hold his breath for up to 8 hours with no negative effects.

Legendary Resistance (3/Day, or 5/Day in Lair). If Xandaltheldrumorr fails a saving throw, he can choose to succeed instead.

Sultan of Sands. When in sandstorms, Xandaltheldrumorr has Advantage on Stealth Checks and can use a bonus action to Hide. When on sandy terrain, he can use a bonus action to burrow. While tunneling, Xandaltheldrumorr does not disturb the ground he moves through, and is considered to have Total Cover. Any attack he makes from underground has Advantage.

Hunter's Mind. Xandaltheldrumorr has Advantage on any Attacks against a target with Prey Mind, and has Advantage on saving throws against being Charmed, Frightened, or put to Sleep. Any failed attempts to read his mind will cause 2d10 Psychic damage.

Multiattack. Xandaltheldrumorr can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws. He can replace his Claw Attacks with Spellcasting or a Tail Attack.

Bite. Melee Attack Roll: +15 to hit, reach 5 ft., one target. Hit: 19 (2d10+8) Piercing damage plus 7 (2d6) Acid damage.

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Frightful Presence. Each creature of the Xandaltheldrumorr’s choice that is within 120 feet and aware of him must succeed on a DC 20 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 Frightful Presence for the next 24 hours.

Acid Crystal Breath (Recharge 4-6). Xandaltheldrumorr exhales a stream of acidic crystal shards acid in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 8d8 piercing damage and 4d8 acid damage on a failed save, or half as much damage on a successful one. On a failed save, a creature is both blinded and bleeding for one minute, and their AC is reduced by one. At the end of its turn, a creature may make a Constitution Saving Throw to try and end one of the ailments. The loss to Armor Class can only be reversed at a Smithy, and four consecutive losses will render the armor unusable.

Sandstorm (1/Day). Xandaltheldrumorr creates a vortex of violently churning sand at a point within 120 feet. All creatures within a 60-foot radius of the point take 1d6 points of slashing damage whenever they enter or begin their turn within the storm. Characters in the swirling sands are blinded and must make a DC 15 Dexterity saving throw whenever they attempt to move more than five feet within the storm, or else fall prone. The terrain within the sandstorm is transmuted into sand, all water is destroyed, and all plant growth withers and dies.

Spellcasting. Xandaltheldrumorr is a natural spellcaster, and can cast the following spells, requiring no Material components and using Charisma as his spellcasting ability (spell save DC 22, +14 to hit):

At Will: Mage Hand, Magic Stone, Minor Illusion, Primal Savagery, Vicious Mockery, Cause Fear, Chromatic Orb, Command, Create or Destroy Water, Earth Tremor, Magic Missile, Tasha's Caustic Brew, Cloud of Daggers, Dust Devil, Earthbind, Invisibility, Melf's Acid Arrow, Ashardalon's Stride, Counterspell, Erupting Earth, Fear, Meld Into Stone, Wall of Sand, Confusion, Dominate Beast, Polymorph, Stone Shape, Stoneskin, Vitriolic Sphere, Conjure Elemental, Hold Monster, Transmute Rock, Wall of Stone

2/Day: Bones of the Earth, Disintegrate, Eyebite, Flesh to Stone, Investiture of Stone, Move Earth, Sandstorm, Mirage Arcane, Power Word: Pain, Project Image

1/Day: Control Weather, Earthquake, Feeblemind, Power Word: Stun, Tectonic Fracture, Volcanic Wrath

Venator's Gaze. When another creature attempts an attack, Xandaltheldrumorr affixes them with a ravenous stare. The creature must make a DC18 Wisdom Check. If they fail, they become afflicted with Prey Mind for one minute. The can repeat the saving throw at the end of each turn. If they succeed, they become immune to Imperious Gaze for the next 24 hours.

A creature afflicted with Prey Mind is compelled to flee as far as their movement speed allows from the creature that caused it, and cannot make any Actions except to effect escape or concealment. If they move more than 500 feet from Xandaltheldrumorr, they regain control of their movements.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature's turn, Xandaltheldrumorr can expend a use to take one of the following actions. He regains all expended uses at the start of each of his turns.

Tunnel. Xandaltheldrumorr burrows into the earth, vanishing from sight. He can then move up to half his burrowing speed.

Deep Ambush. While burrowing within 10 feet of the surface, Xandaltheldrumorr erupts (without spending movement) into a 20-foot square on the ground directly above him. Each creature within the area must make a DC 21 Dexterity Saving Throw. On a failed save, a creature takes 3d8 bludgeoning damage and is Restrained by rubble. On a successful save, they take half damage and are not restrained. A restrained creature can use its action to free itself or another creature within its reach. Success Failure or Success: The target is pushed 5 feet to an unoccupied space of Xandaltheldrumorr's choice, and the area he appears in is Difficult Terrain. Xandaltheldrumorr cannot take this action action again until the start of his next turn. 

Earth Burst. Xandaltheldrumorr casts Erupting Earth. He cannot use this action again until the start of his next turn.

Vicious Lunge. Xandaltheldrumorr leaps up to half his movement speed and makes a Bite Attack.

Tail Sweep. Xandaltheldrumorr sweeps his tail in a 30-foot arc behind him. All creatures in its path must make a DC 18 Dexterity Saving Throw. On a failed save, they take 17 (2d8 + 8) Bludgeoning damage and are knocked Prone. On a success, they take half damage and are not knocked prone.