Amphibious. The dragon can breathe air and water.

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

Spellcasting. The Starlight Dragon is a spellcaster. Its spellcasting ability is Wisdom (spell save DC 18, +7 to hit with spell attacks). The Starlight Dragon has following spells prepared:
1st level (3 slots):  Magnify Gravity, Guiding bolt
2nd level (2 slots): Flaming sphere, hold person
3rd level (1 slots): Pulse Wave, daylight

Multiattack. It then makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 6) piercing damage and (2d6) radiant damage. 

Claw. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) slashing damage and (2d6) radiant damage.

Tail. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 15 (2d8 + 6) bludgeoning damage and (2d6) radiant damage.

Breath Weapons (Recharge 5–6). The dragon uses one of the following breath weapons.

Starfire Breath. The dragon exhales starfire in a 90-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 24 (4d10) fire damage and 24 (4d10) radiant damage on a failed save, or half as much damage on a successful one.