Pirates from the Deep
There are different versions of the legend, naming different races for the pirates from the deep. Some say they were merfolk, some say tritons and others say that they are an unknown race which never interacts with sentient beings other than to kill and steal from them. In short, there is a version of the legend about any water breathing races and some of them, like the tritons, are very offended upon hearing the stories. However, all versions of the legend agree that the pirates from the deep would most often strike at nighttime and typically during cloudy or foggy weather. They'd silently rise up from the water, climbing on board the ship and assassinate the night watchers. They'd collect food, treasures, weapons and anything they liked and take it with them. If anyone caught them in the act, the person would be killed and once they returned to the water they would bring the dead with them for reasons unknown. Perhaps as another food source. The legend was first spread by sailors who claim to have woken up during the silent scavenging but managed to hide or trick the pirates that they were still sleeping or they would surely have been killed as well.
It is thought that the pirates of the deep did exist but no concrete proof has been found of what they were, where they came from or what they did with their spoils. Only a few witness accounts have been deemed credible enough, though it is still difficult to discern fact from fiction. Especially since the stories became popular very quickly and bards all over had their own renditions of the tale already during the years that the pirates of the deep were thought to be active.