Ashardalon

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

Terrifying Presence. Any creature that starts its turn within 160 feet of Ashardalon and is aware of him must succeed on a DC 25 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 has advantage on saving throws against Ashardalon’s Frightful Presence for the next 12 hours.

Fiendish Fury (Mythic Trait; Recharges after a short or long rest). When Ashardalon would drop to 0 hit points, his hit point total instead resets to 550, he recharges his Legendary Resistance and Fire Breath, and immediately affects any creature within 120 feet with Terrifying Presence. In addition, he can use his Mythic Actions as legendary actions for 1 hour. Award an adventuring party an additional 135,000 XP (270,000 XP total) for defeating Ashardalon after this trait is activated.

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: +19 to hit, reach 20 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 17 (5d6) force damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 19 (2d8 + 10) slashing damage and the target is grappled (escape DC 27). Ashardalon can grapple two creatures at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., one target. Hit: 21 (2d10 + 10) bludgeoning damage, and if the target is Large or smaller, it is knocked prone.

Fire Breath (Recharge 5–6). Ashardalon exhales fiendish fire in a 350-foot cone. Each creature in that area must make a DC 27 Dexterity saving throw, taking 84 (24d6) fire damage and 21 (6d6)necrotic damage on a failed save, or half as much damage on a successful one.

Abyssal Strength (1/day). Ashardalon calls on demonic power. For 1 minute, he deals an extra 4 (1d8) necrotic damage with his attacks, and he regains 10 hitpoints at the start of his turn is he has at least 1 hit point, and any creature that hits him with a melee attack takes 7 (2d6) necrotic damage.

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.

Tail Attack. The dragon makes a tail attack.

Wing Attack (Costs 2 Actions). The dragon beats its wings. Each creature within 25 feet of the dragon must succeed on a DC 27 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

If Ashardalon's mythic trait is active, he can use the following Mythic Actions as legendary actions for 1 hour after using Fiendish Fury. 

Bite Attack. Ashardalon makes a bite attack.

Fiendish Flames (Costs 3 Actions). Ashardalon flares with abyssal flames. Each creature within 40 feet of him must make a DC 27 Constitution saving throw, taking 14 (4d6) necrotic damage and 17 (5d6) fire damage on a failed save, or half damage on a success.