Ancient Sun Dragon

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

Innate Spellcasting.

The dragon’s innate Spellcasting Ability is Charisma (spell save DC 22). It can innately cast the following Spells, requiring no material components:

At will: heat metal, light, prismatic spray

Collapsing Star.

When an ancient sun dragon dies, its body collapses in on itself, creating a sphere of annihilation (60% chance) with a 30 foot diameter, or a well of many worlds (40% chance) with a 25 foot diameter. In either case the phenomenon is unstable. At the beginning of each day, roll 1d100. On a result of 5 or less the phenomenon dissipates.

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: 20 (2d10 + 10) piercing damage plus 12 (4d6) fire damage.

Claw. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 14 (2d6 + 8) slashing damage. 

Frightful Presence. Each creature of the dragon's choice that is within 120 ft. of the dragon and aware of it must succeed on a DC 21 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.

Solar Breath (Recharge 5–6).

The dragon exhales a small, slow moving fireball to a space within 90 feet of it. The fireball lingers in place, hovering and emitting 90 ft. of bright sun-light. As a reaction, any time another creature starts or ends its turn, the dragon may force all lingering fireballs to explode, searing the 60 feet around each fireball with solar energy. Each creature in that are must make a DC 24 Dexterity saving throw, taking 49 (14d6) fire damage and 49 (14d6) radiant damage on a failed save, or half as much on a successful one.

Legendary Actions

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 their turn.

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

Disengage. The dragon takes the disengage action, and it cam move up to half its fly speed.

Cast a Spell (Costs 2 Actions). The dragon casts a spell it can cast innately.