Guardian Naga

Large monstrosity, lawful good

Armor Class 18 (natural armor)
Hit Points 127 (15d10 + 45)
Speed 40 ft.

Saving Throws Dex +8, Con +7, Int +7, Wis +8, Cha +8
Damage Immunities poison
Condition Immunities charmed, poisoned
Senses darkvision 60 ft., passive Perception 14
Languages Celestial, Common
Challenge 10 (5,900 XP)

Rejuvenation. If it dies, the naga returns to life in 1d6 days and regains all its hit points. Only a wish spell can prevent this trait from functioning.
Spellcasting. The naga is an 11th-level spellcaster. Its spellcasting ability is Wisdom (spell save DC 16, +8 to hit with spell attacks), and it needs only verbal components to cast its spells. It has the following cleric spells prepared:
Cantrips (at will): mending, sacred flame, thaumaturgy
1st level (4 slots): command, cure wounds, shield of faith
2nd level (3 slots): calm emotions, hold person
3rd level (3 slots): bestow curse, clairvoyance
4th level (3 slots): banishment, freedom of movement
5th level (2 slots): flame strike, geas
6th level (1 slot): true seeing

Actions

Bite. Melee Weapon Attack: +8 to hit, reach 10 ft., one creature. Hit: 8 (1d8 + 4) piercing damage, and the target must make a DC 15 Constitution saving throw, taking 45 (10d8) poison damage on a failed save, or half as much damage on a successful one.
Spit Poison. Ranged Weapon Attack: +8 to hit, range 15/30 ft., one creature. Hit: The target must make a DC 15 Constitution saving throw, taking 45 (10d8) poison damage on a failed save, or half as much damage on a successful one.