The Deep Devils were a race of aquatic beings that attacked Bulwar, Kheterata, Fangaula and other regions along the coast of the Divenhal in the period between -1200 BA and -1134 BA, known as the Onslaught, that caused the decline and collapse of several Sarhaly civilizations and according to some scholars, triggered the rise of non-human civilizations, like harpies or gnolls.
There are fragmentary records of the Deep Devils, mostly myths, since the invasion triggered a Dark Age from which we hardly find written records of the immediate period after the Onslaught. Their appearance is currently unknown, and several cultures depicted them in a different fashion, which led the scholars to think that the Deep Devils may be a confederation of aquatic beings, rather than a single species.
Other scholars, like Jahan Síl Bennon prior to his descent, disregards the Deep Devils as a myth to explain the fall of ancient civilizations due to internal causes, as he wrote on a letter to Amarien Heartseeker “It’s hard to believe that such civilizations could fall to them, I mean, [sic] it's just fish and octopus people n shit.”