Caelivorax, Paragon of the Astral Skies

Frightful Presence. Each creature of Caelivorax’s choice that is within 120 feet of the serpent and aware of it 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 the serpent’s Frightful Presence for the next 24 hours.

Mithril Scales. For the attacks this creature takes anywhere but to its head, halve the damage taken.

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

Multiattack. The serpent can use its Frightful Presence. It then makes three attacks: one with its tail and two with its bite. It may use the other actions once per turn.

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d20+10) piercing damage.

Gargantuan Tail Attack. Melee Weapon Attack: +16 to hit, reach 200 ft., three targets. Hit: 16 (1d20+10) bludgeoning damage. 

Hyperbeam (Recharge 4-6). Caelivorax fires a death ray from his mouth. Ranged Attack: +16 to hit, reach 150 ft., single target. Hit: 16 (4d20+60) force damage.

Channel Astral Force. Use your action to gain resistance to all damage types, immunity to all status conditions and cure any status conditions that Caelivorax has for 2 turns.

Sunbeam. A beam of brilliant light flashes out from your mouth in a 5-foot-wide, 60-foot-line. Each creature in the line must make a Constitution saving throw. On a failed save, a creature takes 6d8 radiant damage and is blinded until your next turn. On a successful save, it takes half as much damage and isn't blinded by this spell. (Save: CON 20)

Mithril Reflection. If the Caelivorax makes a successful saving throw against a spell, or a spell attack misses it, the serpent can choose another creature (including the spellcaster) it can see within 30 feet of it. The spell targets the chosen creature instead of the serpent. If the spell forced a saving throw, the chosen creature makes its own save. If the spell was an attack, the attack roll is rerolled against the chosen creature.

Caelivorax has 1 legendary action which resets every turn.

Mithril Blood. Recover 2d20 hit points. Any creature to come in contact with Caelivorax next turn will be poisoned.

Cosmic Roar. Pick a creature of your choice in 50 ft. of Caelivorax's head. The chosen creature must roll a 20 WIS saving throw or become frightened.