Ancient Luminous Dragon

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.

Empty 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.

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.

Light That Burns the Sky. Recharge: 10  Each creature within 120 feet of the dragon must make a DC 23 Dexterity saving throw or take 100 (20d10) radiant damage, or half as much on a successful save. They must then make a DC 23 Constitution saving throw or be blinded until magic such as greater restoration or a wish removes the blindness. On a successful save, the targets are immune to being blinded for 1 minute.

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.

Tail Attack. The dragon makes one Tail attack.

Burst of Light (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.