Ancalagon The Black

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Wrath (1/day). If Ancalagon would be reduced to 0 hit points, his current hit point total instead resets to 100 hit points, he recharges his Fire Breath, and he regains 1 expended use of Legendary Resistance. Additionally, Ancalagon can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 15,000 XP 

Darkness (Always). Ancalagon the Black is a creature of immense magical power and darkness. Some of this power has diffused from him into the surroundings. There is a permanent nonmagical darkness effect surrounding Ancalagon, up to 100 feet away. This darkness can be dispelled by dispel magic, anti-magic field, or the Wish spell. It can also be seen through with ordinary light. 

Duration: Always

The Non-magical darkness spreads to fill a 100ft radius sphere centered on Ancalagon for the duration. The darkness spreads around corners. 

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 11 (3d6) fire damage.

Claw. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit:  17 (2d8 + 8) slashing damage.

Frightful Presence. Each creature of the dragon's choice that is within 180 feet of the dragon and aware of it must succeed on a DC 19 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 the dragon's Frightful Presence for the next 24 hours.

Fire Breath (Recharge 5-6). The dragon exhales fire in a 90-foot cone. Each creature in that area must make a DC 19 Dexterity saving throw, taking 110 (20d10) fire damage on a failed save, or half as much damage on a successful one.

Let Darkness Reign. If Ancalagon's darkness effect has been dispelled, He can use a bonus action to send a pulse of darkness that grows 50ft per round around him. The darkness stops advancing once it has reached its full 100ft radius.  

Mind Sliver (3/day). Ancalagon loathes the presence of magic that is not his own and wishes it to fail. 3 times a day, Ancalagon can innately cast Mind Sliver, targeting any servant of a deity that is not him. 

Enchantment cantrip

Casting Time: 1 action
Range: 60 feet
Components: V
Duration: 1 round

Ancalagon drives a disorienting spike of of his will into the mind of one creature he can see within range. The target must succeed on an Intelligence saving throw or take 5d6 psychic damage and subtract 1d4 from the next saving throw it makes before the end of his next turn.

The dragon 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. The dragon regains spent legendary actions at the start of its turn.

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

Bite Attack. The dragon makes a Bite attack. If the target is a creature, it is grappled (escape DC 19). Until this grapple ends, the target is restrained, and Ancalagon can’t bite another target.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 40 feet of the dragon must succeed on a DC 19 Dexterity saving throw or take 25 (3d8 + 12) bludgeoning damage and be knocked prone. Ancalagon can then fly up to half its flying speed.

Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit:  19 (2d10 + 8) bludgeoning damage.

Mythic Actions
If Ancalagon's Wrath trait has activated in the last hour, he can use the options below as legendary actions.

Swallow. Ancalagon makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target 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 45 (10d8) acid damage at the start of each of Ancalagon's turns.

If Ancalagon takes 40 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 10 feet of the Ancalagon. The creatures continue to take 2d6 acid damage until they spend an action wiping off the remaining acid.  If Ancalagon dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.