Olyhydra Chapel Tactical Battle

Setup

Canon Belsornig (D)
Undertow Wight (W)
3 Fathomers (85 hit points) +1 per four PCs (B)

Tactics

Preferring to remain at a distance, Belsornig moves to the south to use venomous words while he hides behind the advancing fathomers. If he is forced into melee, he uses dark dagger to slide an opponent into a cell, then slams the door shut to lock it (see "Features of the Area"). He will silence any obvious spell casters and is not adverse to healing his allies. He will have resist fire active at the start of the battle. If Belsornig has not had a chance to prepare (for example, he is surprised) he gains 1 legendary action per PC which he may expend at the end of each PCs turn until all of the following has occurred - in order:

He summons a water elemental bodyguard.
He summons a spiritual hammer.
He casts energy resistance: fire on himself.
He casts silence 15 feet on an obvious spell caster.

The fathomers turn into large snakes made of water and use leap (see below) to stay in front of the adventurers, keeping foes away from Belsornig. They have armor of Agathys active and can cast spells in snake form:

Leap (Recharge 6). While in snake form, the fathomer jumps up to half its speed, gaining a +5 bonus to all AC and saving throws during this movement. It then attacks with its bite attack that does 2d6+6 damage or 3d6+6 if it has advantage on the attack roll.

Sister Philla and Brother Abramo (undertow wights) have given their lives in the service of Olhydra, and are now undertow wights (a type of deathlock covered in seaweed and barnacles dripping seawater constantly). They use horrific visage before the fathmoers enter combat and reanimate each other if they fall in battle. If any adventurer enters a square next to a shrine, the wights try to immobilize that character with grave bolt. The fathomers run if Belsornig is killed; otherwise, all the enemies here fight to the death.

Features of the Area

Light: Lanterns fill the area with bright light.
Corrupted Shrines: These shrines to the Law bringer have been corrupted to serve the needs of the Cult of Evil Water. Any creature that does not worship Olhydra take a -2 penalty to saving throws while adjacent to a shrine. In addition, unbelievers in the room are also exposed to a rancid air effect. Anytime a non-believer receives healing the target must make a DC 15 WIS save or become weakened until they move out of the rancid area. If a shrine is cleansed (DC 15 Religion check), it provides the cleanser with a burst of healing (1d8), removing the weakened state and granting all non-believers advantage on the rancid air saving throw. If both shrines are cleansed, the rancid air effect is cancelled. On each shrine are carved the words "Giver of Law, may your word be established in all worlds." It is common knowledge among followers of both religions that this maxim is spoken before any formal prayer is uttered to the Lawbringer.
Relic Shelves: This large set of shelves mostly contains mundane items, but any search reveals two vials of holy water.
Meditation Cells: The doors to these cells are open unless a cell contains prisoners. A DC 13 Thieves Tool check (a rogue or locksmith will automatically know this on checking) reveals that the normal locks have been reversed to operate from the outside and that the doors lock automatically if shut. A door requires a DC 20 Thieves Tool check to open if locked.

Treasure

Belsornig wears a robe of plain green velvet, with a gold belt set with lapis and malachite (jewellery value 750 gp). In his purse hanging from this belt are 21 gp and 14 pp, along with an iron key upon which the word "D'LYSS" is inscribed with runes. This word is the command to stop the juggernaut in the temple proper (original 1e reference: area 213), and the key opens his personal chest in one of the meditation chambers. He wears a gold ring with a peridot bezel (jewellery value 1,000 gp).