Coral Guardian

Regrowth. The guardian regains 10 hit points at the start of its turn. If the guardian takes acid or necrotic damage, this trait doesn't function at the start of the guardian's next turn. The guardian dies only if it starts its turn with 0 hit points and doesn't regrow.

False Appearance. While the guardian remains motionless, it is indistinguishable from ordinary coral.

Magic Weapons. The guardian's weapon attacks are magical.

Innate Spellcasting. The guardian's innate spellcasting ability is Wisdom (spell save DC 15). The guardian can innately cast the following spells, requiring no material components:
at will: detect evil and good, light(self only), shape water, tree stride(works on coral, coral only)
3/day each: create or destroy water, cure wounds, water breathing
1/day each: daylight(self only), plant growth(works on coral, coral only), remove curse

Multiattack. The Guardian makes two slash attacks.

Slash. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 13 (2d8 + 4) slashing damage. 

Purifying Radiance(Recharge 4-6). The guardian bares its core, releasing a wave of purifying radiance. Each aberration, fiend, and undead within 20 feet of the guardian must make a DC 15 Wisdom saving throw. On a failure, a target takes 17 (3d8+4) radiant damage and is frightened of the guardian. On a success, a target takes half as much radiant damage and isn't frightened. A frightened creature may repeat the saving throw at the end of each of their turns, ending the effect on a success.