Millenia ago, people lived on Loth Ravenor, and stood sometimes for days in the surf and listened to the secrets that the ocean told them. They built a black temple on the island there and killed black sharks and hung them from the walls of the temple, and made instruments from the flesh of sharks and cattle and played old songs that the ocean told them, and prayed, and in time Uilleax the Fathom Wyrm came swimming from under the waves and crawled into the temple and became their god. It taught them what ways to honor it, and spells no one knew, and how to placate the Gods of the Winds and of the Currents, and how to see into the dreams of fish and waves, and make congress with the fish, and be knowledgeable of the ocean. They built Loth Armanea and every millennium Uilleax destroyed one third of their city as payment and sacrifice for the gifts he gave them. One thousand and one years ago, he entered Loth Armanea, set ablaze the city, turned the city council into salt, swallowed the emperor and his nine royal children and ninety wives, and crawled into the ocean, and has not been seen since.
It is said that Uilleax the Fathom Wyrm was three kilometers in length and possessed one thousand legs and was the color of blood and mucous and that his gaze gave strange life to dreams and that in his presence the ocean sang odd arias, and danced.
One thousand and one years now has the temple on Loth Ravenor stood empty and its halls are kept by black puddings, millipedes, serpents, goatmen, and shoggoth. Goats live in the trees around the temple and their horns grow strange, and they speak the language of men and fish. The gates to the temple are guarded by old devotees to the Fathom Wyrm and suffer no one to pass, and still place gold and sacrifice within the entrance where Uilleax once went, and the treasures are taken by the monsters within, for the clerics believe the monsters are beloved of the dragon, and sacred to him.
They pray for his return, and seek him on pilgrimage across the ocean, and Loth Armanea arms itself in preparation.
Clerics of the Fathom Wyrm may choose these powers at the appropriate level instead of their domain power.
1st level: Learn the Language of Fish. Gain proficiency at Swimming, and add double proficiency bonus to Swim checks.
1st level: Your touch becomes poison. Make an unarmed attack as a bonus action. On a hit, the target becomes poisoned for 1d4 rounds (disadvantage on attack rolls and ability checks), and each round that it is poisoned takes 1d4 poison damage. This leaves a suppurating wound, and if the creature takes 10 or more damage this way, the wound scars permanently. You may do this a number of times per day = Wisdom modifier.
2nd level: Channel Divinity: Brine. You can present your holy symbol and cause a salt wind to howl. Each hostile creature within 30 feet of you must make a Constitution saving throw. A creature takes dehydration damage equal to 2d10+cleric level on a failed save, and half that on a successful save. A creature reduced to 0 hp by this power turns to a salt statue. Also on a failed save, each creature's weapons and armor degrades by an amount equal to your Wisdom bonus.
6th level: Channel Divinity: Charm Fish, and Slimes too. You present your holy symbol and speak an invocation to Uilleax Each fish, jellyfish, slime, or ooze that can see (or sense) you within 30' must make a Wisdom saving throw. On a failure, it becomes charmed by you for 1 minute or until it takes damage. The charmed creature is friendly to you and anyone else you decide
8th level: Once on each of your turns when you hit a creature with your weapon you may deal an extra 1d8 psychic damage as their thoughts become torturous and terrifying. When you reach 14th level, this (of course) increases to 2d8 damage. For every eight damage a creature takes from this attack, they become unable to sleep for one day.
17th level: You learn the names of the Currents and the Winds. As an action you may name a Wind or an Ocean Current and direct it as you like. A wind may blow in any direction, gust at the strength of a storm, become still, or anything else a Wind might do. An Ocean Current may change directions, become still, become confused, loop onto itself, or the like. Maintaining control over a Wind or a Current counts as maintaining concentration on a spell, and must be spoken to again each round as a bonus action.