Ancient Solar Dragon

Gargantuan Dragon, Neutral

AC: 18

Initiative: +2 (12)

HP: 425 (23d20 + 184)

Speed: 30 ft., fly 120 ft. (hover)

STR: 28 (+9)
DEX: 15 (+2)
CON: 26 (+8)
INT: 17 (+3)
WIS: 18 (+4)
CHA: 16 (+3)

Saving Throws: Dex +9, Con +15, Wis +11, Cha +10

Skills: Perception +18, Stealth +9

Immunities: radiant

Condition Immunities: blinded

Senses: darkvision 240 ft., Passive Perception 28

Languages: Draconic

CR: 21 (XP 33,000; PB +7)

Flyby.

The dragon doesn't provoke opportunity attacks when it flies out of an enemy's reach.

Legendary Resistance (3/Day).

If the dragon fails a saving throw, it can choose to succeed instead.

Nebulous Thoughts.

Magical attempts to read the dragon's mind or glean its thoughts fail automatically.

Siege Monster.

The dragon deals double damage to objects and structures.

Unusual Nature.

The dragon doesn't require air.

Actions

Multiattack.

The dragon makes one Bite attack and one Tail attack.

Bite.

Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 19 (3d6 + 9) piercing damage plus 10 (3d6) radiant damage.

Tail.

Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 13 (1d8 + 9) bludgeoning damage.

Photonic Breath (Recharge 5-6).

The dragon exhales a flashing mote of radiant energy that travels to a point the dragon can see within 240 feet of itself, then blossoms into a 40-foot-radius sphere centered on that point. Each creature in the sphere must make a DC 23 Constitution saving throw, taking 66 (12d10) radiant damage on a failed save, or half as much damage on a successful one.

Legendary Actions

Legendary Action Uses: 3. Immediately after another creature's turn, this creature can expend a use to take one of the following actions; it regains all expended uses at the start of each of its turns.

Tail Attack.

The dragon makes one Tail attack.

Blinding Brilliance (Costs 2 Actions).

The dragon emits magical light in a 30-foot-radius sphere centered on itself. Each creature in this area must succeed on a DC 23 Wisdom saving throw or be blinded until the end of its next turn.

Regional Effects

A solar dragon customarily lairs in a star's radiant core or nests in a gigantic, hollowed-out rock engulfed by the star's blinding radiance. A star in which a solar dragon makes its lair has the following effects on creatures:

Blinding Radiance.. A creature that is within 10 miles of the star or inside it is blinded by its intense light unless the creature has protective gear to shield its eyes, such as goggles of night or a similar form of eyewear.
Stellar Incineration.. Any creature that enters the star or starts its turn inside it takes 132 (24d10) radiant damage.

Find this creature in the encounter builder

Source: Boo's Astral Menagerie, p. 50 - Licenses and attribution