Huge celestial, neutral
Armor Class: 18 (natural armor) Hit Points: 225 (18d12 + 108) Speed: 50 ft., swim 60 ft., fly 30 ft. (hover)
STR: 24 (+7) DEX: 14 (+2) CON: 22 (+6) INT: 12 (+1) WIS: 18 (+4) CHA: 16 (+3)
Saving Throws: DEX +7, CON +12, WIS +9 Skills: Perception +9 Damage Resistances: radiant Senses: blindsight 60 ft., passive Perception 19 Languages: Celestial, telepathy 120 ft. Challenge Rating: 15 (13,000 XP)
Aura of Radiance: The serpent emits an aura of radiant energy within a 30-foot radius. Creatures of the serpent's choice that start their turn in the aura take 15 (3d8 + 2) radiant damage. Undead and fiends have disadvantage on saving throws against this damage.
Innate Spellcasting: The serpent's spellcasting ability is Wisdom (spell save DC 17). It can innately cast the following spells, requiring no material components:
At will: light, daylight 3/day: moonbeam, sunbeam 1/day: prismatic spray
Legendary Resistance (3/Day): If the serpent fails a saving throw, it can choose to succeed instead.
Actions
Multiattack: The serpent makes two attacks: one with its bite and one with its tail.
Bite: Melee Weapon Attack: +12 to hit, reach 15 ft., one target. 8d8 + 20 radiant damage.
Tail: Melee Weapon Attack: +12 to hit, reach 20 ft., one target. 8d8 radiant damage. The target must succeed on a DC 18 Strength saving throw or be knocked prone.
Radiant Breath (Recharge 5-6): The serpent exhales a radiant breath in a 60-foot cone. Each creature in that area must make a DC 18 Dexterity saving throw, taking 54 (12d8) radiant damage on a failed save, or half as much damage on a successful one. Creatures that fail their saving throw are also blinded until the end of their next turn.
Sea of Light Glide: The serpent can move across the surface of the Sea of Light as if it were solid ground.
Reactions
Radiant Deflection: When the serpent is hit by a ranged spell attack, it can use its reaction to impose disadvantage on the attack roll. If the attack misses, the serpent gains temporary hit points equal to the spell's level + its Charisma modifier (minimum of 1 temporary hit point).
Legendary Actions
The serpent can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time, and only at the end of another creature's turn. The serpent regains spent legendary actions at the start of its turn.
Burst of Radiance: The serpent releases a burst of radiant energy. Each creature within 20 feet of the serpent must make a DC 17 Constitution saving throw, taking 10d8 radiant damage on a failed save, or half as much damage on a successful one.
Radiant Strike: The serpent makes a bite attack.
Luminous Flight: The serpent uses its movement to fly up to its speed without provoking opportunity attacks.
Radiant Awe: Everyone must make a DC15 Con check or be afraid
Serpent Strike: The Serpent can give itself a 20 for its initiative roll