Ancalagon, The black

Empowered Strikes. Ancalagon's attacks attacks are treated as magical and adamantine for the purpose of bypassing resistance and immunity to nonmagical weapons.

Freedom of Movement: Ancalagon ignores difficult terrain, and magical effects can't reduce his speed or cause him to be restrained. He can spend 5 feet of movement to escape from nonmagical restraints or being grappled. 

Impenetrable Scales. Each time that Ancalagon takes damage if the total damage isn't 30 or higher then Ancalagon takes 0 damage instead.

Innate Spellcasting. Ancalagon's innate spellcasting ability is Charisma (spell save DC 24, +16 to hit with spell attacks). He can innately cast the following spells, requiring no material components:

At will: suggestion

1/day each: control weather, earthquake, true seeing

Legendary Resistance (5/Day). If Ancalagon fails a saving throw, he can choose to succeed instead.

Siege. Ancalagon's attacks deal double damage to objects and structures.

Titanic Behemoth. Ancalagon is so large that he's immune to most conditions of effects created by things smaller then him. Ancalagon automatically succeeds on saving throws to resist any spell or magical effect not created by a Gargantuan creature. Additionally Ancalagon is immune to being Charmed, Deafened, Exhaustion, Frightened, Grappled, Incapacitated, Petrified, knocked Prone, Restrained, Stunned, or knocked Unconscious by any creature that's smaller then Gargantuan.

Multiattack. Ancalagon can use his Frightful Presence. He then makes three attacks: one with his bite and two with his claws.

Bite. Melee Weapon Attack: +19 to hit, reach 45 ft., one target. Hit: 54 (8d10 + 10) piercing damage plus 35 (10d6) fire damage. If the target is a Gargantuan or smaller creature, that creature is grappled (escape DC 27). While grappled in this way a creature is restrained. A creature that starts its turn grappled in this way takes 32 (4d10 + 10) piercing damage plus 17 (5d6) fire damage. Ancalagon can't make a bite attack against another target while it has a creature grappled this way.

If the target is a Medium or smaller creature grappled by Ancalagon, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Ancalagon, and it takes 35 (10d6) fire damage at the start of each of Ancalagonss turns.

If Ancalagon takes 50 damage or more on a single turn from a creature inside it, Ancalagon must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 15 feet of Ancalagon. If Ancalagon dies, a swallowed creature is no longer restrained by him and can escape from the corpse using 150 feet of movement, exiting prone.

Claw. Melee Weapon Attack: +19 to hit, reach 30 ft., one target. Hit: 38 (8d6 + 10) slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 27). If the target is a Gargantuan creature then it must make a a DC 27 Strength saving throw or be grappled. While grappled in this way a creature is restrained.

Tail. Melee Weapon Attack: +19 to hit, reach 60 ft., one target. Hit: 46 (8d8 + 10) bludgeoning damage. If the target is a Huge or smaller creature, it is knocked prone. If the target is a Gargantuan creature then it must make a a DC 27 Strength saving throw or be pushed 60 feet back.

Frightful Presence. Each creature of Ancalagon's choice that is within 1 mile of him and is aware of him must succeed on a DC 27 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 Ancalagon' s Frightful Presence for the next 24 hours.

Stomp. Ancalagon slams the earth with his mighty claws causing a cataclysmic shock wave, each creature within 300 ft. of Ancalagon must make a DC 27 Strength Saving Throw, on a failure a creature takes 44 (8d10) bludgeoning damage plus 28 (8d6) piercing damage and is flung 80 feet back and knocked prone. On a success a creature is not flung back and takes half as much damage. If a thrown target strikes an object, such as a wall or floor, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 15 Dexterity saving throw or take the same damage and be knocked prone. A creature takes half damage and isn't knocked prone on a success.  

Cataclysmic Breath (Recharge 5–6). Ancalagon exhales fire in a 600-foot cone. Each creature in that area must make a DC 27 Constitution saving throw, taking 210 (20d20) fire damage on a success, or be reduced to 0 hit points on a failure. A creature with Legendary actions or Legendary resistances automatically succeeds on this save. If Ancalagon's breath reduces a creature or object to 0 hit points, the target is reduced to ash. A creature reduced to ash can be restored to life only by a true resurrection or wish spell.

Ancalagon 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. Ancalagon regains spent legendary actions at the start of his turn.

Detect. Ancalagon makes a Wisdom (Perception) check.

Tail Attack. Ancalagon makes a tail attack.

Ferocious Strike (Costs 2 Actions). Ancalagon uses his stomp.

Wing Attack (Costs 2 Actions). Ancalagon beats his wings. Each creature within 120 feet of Ancalagon must succeed on a DC 27 Dexterity saving throw or take 52 (12d6 + 10) bludgeoning damage and be knocked prone. Ancalagon can then fly up to half his flying speed.

Fire Breath (Costs 3 Actions). Ancalagon uses his breath attack, but if it isn't charged then he can recharge it.