Under the sea that surrounds the Iksaltari Archipelago there lies both a sprawling and lively aquatic civilization and the underworld that houses the souls of the dead. The seabed and its cities are ruled over by Oreish, Lord of the Deep Castle. There are various communities that have coalesced into population centers but there are far more racial divisions than on land. There are also many nomadic groups that traverse the tides.
Races (non-exhaustive)
Golem
The golems that serve the god Oreish form around the souls of exceptional mortals. They populate the Crystal Palace in bodies of rock, crystal, or coral. Golems do not retain their memories from life but as proof of their undeath they cannot survive out of salt water. They do not age, nor do they have a lifespan. They can, however, be killed. Since golems are so rarely born, this keeps their population relatively stable. For the most part golems faithfully serve their liege, but the occasional independant golem can be found living in towns near the palace or in large population centers. They cannot speak audibly, communicating solely via sign language, but they can understand spoken word. Golems have long lifespans but their memories degrade with time. Some experience hard resets where all their memories are wiped clean every 200 years while others simply overwrite their oldest memories with new ones. Golems usually take simple words as their names.
Crystal/Coral Golem Subrace
Ability Score Increase. Your Charisma score increases by 2.
Necrotic Resistance. You have resistance to necrotic damage.
New Growth. You can regrow the damaged parts of your body over time. Once per long rest, you can regain one hit die of HP as a bonus action.
Aquatic Gnoll
A gnoll subrace adapted to an underwater climate. They shave their fur short to swim at higher speeds.
Ability Score Increase. Your Dexterity score increases by 1.
Amphibious. You can breathe air and water.
Emissary of the Sea. Aquatic beasts have an extraordinary affinity with your people. You can communicate simple ideas with beasts that can breathe water. They can understand the meaning of your words, though you have no special ability to understand them in return.
Apex Shark
Sentient sharks that live outside urban centers. They have a tenuous relationship with other races and are predators of golems.
Languages
Instead of Common, all aquatic creatures know Aquan alongside their racial languages. Amphibious creatures may also know Common depending on their personal history. Any creature that has spent time underwater knows Aquan Sign as well, since many aquatic creatures do not communicate via sound.
Classes
Clerics are exceedingly rare underwater since the gods (with the exception of the reticent Gesem) cannot reach through the waves and Oreish doesn't share his divine power easily. However, Oreish will occasionally make warlock pacts. Most people undersea are not religious. There is a small but mostly harmless cult worshipping krakens.
The Order of the Watch
The guardians of the sea vents. A matrilineal order of mostly related tritons who patrol areas in which aboleths and abyssal creatures might break loose and keep watch on any happenings. The are sworn to protect the ocean from such threats.
Characters include Haivitr, a recently promoted knight; Michal, an unlucky messenger; Bohumil, a mid-ranking officer who needs a vacation; and Draha, an intimidating older woman in charge of dealings with outsiders.