Tritons

"You know, I 'ooked a Triton once! Yah, bastard leapt right out of the water and bit me on the nose! Still got ze scar right 'ere, see?"
- Arulo Kedrisk, probably.

Background

Tritons, sometimes referred to as Seafolk or Deep-Dwellers, are an amphibious race of humanoid creatures who live exclusively in some of the deepest and coldest depths of the ocean. As a result of this, the vast majority of people in Darthas will go their entire lives never even hearing of a Triton let alone seeing one, and on the rare occasion one is spotted (usually by fishermen or other sailors) they are often misconstrued as some kind of Merfolk, rather than a separate species.
The only major settlement of Tritons is found in the largely unexplored reaches and depths of the southern ocean - the great and ancient city of Tritamor clings to the side of a now extinct volcano. Lit by the pale blue glow of a thousand floating Driftglobes, the city is a beacon of light in an otherwise pitch black ocean, and attracts more life than just its native Tritons. Though to a surfacer the city would be inhospitable - a frigid place with temperatures well below zero, barely enough light to see by, not to mention pressure great enough to crush a human's bones - to a Triton there is no place like it. So great is the city considered among its denizens that not one ever saw fit to make another, lest it detract from Tritamors impressive beauty.

Solitary Sea Dwellers

Tritons don't resent that those who live on land (surfacers, as they call them) have almost no idea they exist - in fact quite the opposite is true. Their detachment from the rest of the world both physically as well as culturally allows them to maintain status of strict isolationism, and although it is not forbidden almost no Tritons have ever gone to the surface, even for a cursory visit.
As a result of this, the Seafolk have almost no knowledge of what goes on above the waves. That being said however, The Deepest One does maintain an informant network comprised almost exclusively of different types of cephalopods, who have proven very effective at gathering information from humans on the surface before managing to escape back to the ocean. While most simply think an octopuses natural aptitude for escapism is just a lucky biological trait, it is in reality a result of many years of training as a Triton intelligence agent.

Friends of the Fish

Just as humans and other surface dwelling races have developed an almost symbiotic relationship with many of the creatures who live there, the Tritons have not only done the same with the own aquatic neighbours - but have taken this to its natural extreme. All Triton are born with the ability to communicate simple ideas to fish and other sea creatures, though most have nothing particularly interesting to say. More intelligent sea creatures however are a different story: as mentioned before many cephalopods are intelligent enough to act as intelligence agents on the surface, sharks are used as guards and protectors, while dolphins are used in the same way a surfacer might use a horse.

A Quick Study

Unlike surfacers, who are considered very slow learners, Tritons take quickly to most things they turn their hands to, especially the magical arts. Though the amount of spells and information that can be maintained over time and generations is fairly limited (the fabled 'paper' used by the surfacers has proven much less effective after being submereged), most of their kind have at least a basic understanding of Druidic magic.

The Saltstone Chair

The leader of the Tritons - known as The Deepest One - has always historically sat on the Saltstone Chair: an ancient and ornate throne made entirely of salt, thought to be older even than Tritamor itself. While in the ancient days the Deepest One held almost absolute power, their role has moved back to become more of a figurehead than an absolute ruler, though from a technical perspective they maintain direct control of all Tritamor.
The current Deepest One is Yezhan the 18th, ruling in an unbroken familial line dating back tens of thousands of years to the finding of the Saltstone Chair and the eventual founding of Tritamor

A Mutual Destruction

Despite their staunch isolationism and impressively deliberate attempts to ignore anything occurring on the surface, the impending sludge apocalypse is not something even Tritons could ignore. The vile ooze corrupts the oceans, leaving it inhospitable to even the hardiest of sea life. Though Tritamor itself is protected by magic far too ancient to be overcome so quickly, the situation was seen as dire enough to send an emissary to the surface, a measure not taken in many thousands of years.