Merrow

Faced with the constant threat of Outerfolk remnants dwelling in the deep waters of Aiaos, the Seafolk sought many means of defending themselves, but none so dark as those merfolk who accepted the aid of the Abyss to grant them power.
Initially serving the other seafolk as shock troops, within a handful of generations these 'merrow' became completely feral, their vast size and strength matched by a hunger beyond even that of the sahuagin and their bloody gods, which consumed all reason and loyalty. At last, recognising no kin but others tainted by the same otherworldly corruption, they ravaged and then fled from the sea kingdoms into the deeps, where the call of their dark masters echoed in the roiling murk.
Like all of the seafolk kindreds, the merrow hail from a range of initial stock, not a single proto-race, and are united by the Abyssal taint in their heritage and by devotion to their demon-gods. It is rumoured that merrow can be redeemed if they turn from the cults while some part of their mind remains free of the terrible hunger which curses the kindred, and particularly large Merfolk and Tritons noted for short-tempers and martial prowess may have the reputation of merrow-blood.