marrows
The marrows are a water-dwelling merfolk met with in the drowned halls of the cistern; the guards named them, with some caution, as among the possible tenants of those flooded waters. They are creatures upon whom a Charm Person spell finds no purchase. They fight with harpoons trailing lines, haul themselves onto the boardwalks by their claws and by daggers of improvised make, and are deadliest of all in the water they call home.
Two of them broke the surface to fall upon the company from a hunter's trap. One Merrow struck from the water with its harpoons whilst its fellow clambered toward Miray with claws and a rude dagger; a rock flung by a Bichura caught the first full in the head and drew blood. Sidra's Sleep spell laid the nearer of them low, and the other thought better of the affair, disengaged, and slipped beneath the surface. The survivor was not yet finished, however, for it overtook Miray whilst she wore the shape of a crocodile and mauled her until that form broke apart, only to be wounded in its turn by her discordant whispers and to flee into the murk. One Merrow, in the end, met the Medusa's petrifying gaze and was turned to stone.