Merfolk

One of the most mysterious and least studied races are the Merfolks. Though the term itself is used over a wide range of likely different species that have not yet truly been differentiated the general consensus for merfolks is that it's a blanket term used to describe the intelligent species which inhabit the ocean depths. This can range from half-human half fish mermaids, to aquatic but bipedal fish people. Though sea giants are notably distinct from this categorization. Most information about Merfolks comes from fishermen who sometimes will catch one in their nets. The Merfolks seem to have several underwater cities, though the full extent to what these cities are is still unknown, the most notable ones mentioned being Atlantis, The Abyss,  The Maelstrom  and “The Maw.” In the Southern Sea they are known to interact more with humans traditionally and more often in the north they avoid them at all costs. This may be in part due to the fact it's widely known among sailors that merfolks cause shipwrecks and general bad luck which results in fisherman not being kind to Merfolks whom they catch. 
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The Abyss as described by a Merfolk
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Though not currently translated The Merfolk are one of only a handful of a species with a written language. Many artifacts have washed up on beaches, presumably from the ocean's depths with Merfolk writing carved into them. With many common phrases such as, ኢትዮጵያ
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Female Merfolk in its natural habitat