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Merrows are creatures that look like an humanoid/fish hybrid. They are known to use archaic tools and weapons and live in tribes, suggesting sentience. However, their sentience has not been recognized by any international institution yet.
There are 2 sub-species of Merrow. One lives in the open seas of the world, the other is adapted to freshwater.
It is rumored that some Sea Merrows tribes have developed advanced sapience recently, thanks to the magical intervention of a Great Leviathan.
The freshwater Merrows have been reported in the Tonle Sap, the large lake just south of Angkor, in Cambodia, and in the Mekong river that connects to the Tonle Sap. It is theorized that freshwater Merrows are much more advanced than their regular seafaring cousins because of their close proximity to humanity and its tools. This seems confirmed by a shadowtalk in Shadows of Asia describing armored Merrows with harpoon guns. Since local fish species are also adapted to surviving during the long seasonal drough, one can assume that those Merrows possess the same ability to stay out of water for long periods.