Asteroth, The Great Deciever

Amphibious. The dragon can breathe air and water.

Legendary Resistance (5/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Reflective Carapace. If Asteroth is targeted by a Magic Missle spell or a spell that requires a ranged attack roll, roll 1d6. On a 1–5, Asteroth is unaffected. On a 6, Asteroth is unaffected and reflects the spell, turning the caster into the target.

Siege Monster. Asteroth deals double damage to objects and structures.

Divine Might. Asteroth's attacks and effects ignore the resistances and immunities of creatures CR 25 and below. Asteroth's attacks and effects targeting creatures of CR 26-35 ignore resistances and treat immunities as resistance (condition immunities provide advantage in lieu of immunity.)

Regeneration. Asteroth regains 50 hit points at the start of his turn.

Multiattack. The dragon makes four Rend attacks. It can replace one attack with a use of his Spellcasting.

Rend. Melee Attack Roll: +17, reach 25 ft. Hit: 26 (3d10+10) Slashing damage plus 22 (3d10+6) Poison damage. Huge or smaller creatures hit by Rend are Grappled. Asteroth can only Grapple two creatures at a time. A DC 23 Strength Saving Throw is required to break from the Grapple.

Swallow. Asteroth makes a Rend attack against a creature it has Grappled. On a hit, the creature is swallowed and the Grappled condition ends. (it can have up to six creatures swallowed at a time). A swallowed creature has the Blinded and Restrained conditions and can’t teleport, it has Total Cover against attacks and other effects outside Asteroth, and it takes 56 (16d6) Acid damage at the start of each of Asteroth's turns.

If Asteroth takes 60 damage or more on a single turn from a creature inside it, Asteroth must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, each of which falls in a space within 10 feet of Asteroth and has the Prone condition.

Poison Breath (Recharge 5–6). Constitution Saving Throw: DC 26, each creature in a 300-foot Cone. Failure: 93 (17d10) Poison damage. Success: Half damage.

Spellcasting. The dragon casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 23):

At Will: Detect Magic, Mind Spike (level 5 version), Illusory Dragon, Invisibility, Mirror Image, Major Image
1/Day Each: Geas, Modify Memory, Maze

Legendary Action Uses: 4. Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Mind Invasion. The dragon uses Spellcasting to cast Mind Spike (level 5 version).

Noxious Miasma. Constitution Saving Throw: DC 26, each creature in a 30-foot-radius Sphere centered on a point the dragon can see within 90 feet. Failure: 32 (6d10) Poison damage, and the target takes a −2 penalty to AC until the end of its next turn. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Pounce. The dragon moves up to half its Speed, and it makes one Rend attack.

 When Asteroth would drop to 0 hitpoints, he instead gains 500 hitpoints and recovers from any conditions affecting him. He then recovers his Legendary Resistances and has access to the Mythic Action(s) below.

Cataclysmic Eruption. Asteroth engulfs the battlefield in Noxious Gas before flying 120 feet above the plume. All creatures inside must make a DC 26 Constitution Save or be Poisoned and Incapacitated. At the end of the round, Asteroth drops a gleaming mote of molten flame into the Gas. The Gas ignites into a roaring explosion. Every Creature caught inside must make a DC 23 Dexterity Saving Throw. Failure: 120 (22d10) Fire damage, and any object not being carried takes 220 Fire damage. If anything in the Explosion is reduced to 0 hitpoints, it is turned into a pile of dust. Success: Half Damage.