In Kael's D&D campaign, Erebus was Oghma's vault. It was an ancient world filled with many ancient libraries of lost knowledge. Mammon (the evil God of Mind) wanted this knowledge, and launched a huge assault on this world. He was mostly successful in his conquest, but to protect it Oghma (the neutral Goddess of Knowledge) created a thick mist to cover her world, which had the effect of erasing the memory of anyone but her angels (and maybe even them too, but less readily). This made Mammon's demons forget their purpose there, driving those who stayed here insane. There were pirates (Lanun?) who sailed on the mists instead of seas (or possibly on water hidden by the mists). I believe that the Calabim were also present in this game, and that Prespur of the Storms was located near an eternal Maelstrom that served as a passage into the world. I think that humanity entered this plane after the war (or maybe the inhabitants were just people who died and didn't know it), but some "humans" were actually descended from Oghma's angels. Lita the Witch was one of these people. When she was cast down the Cliffs of Hastur, she was taken by one of Mammon's strongest demons (if it was Hastur himself, then it was his Archangel) ad forced to give birth. This was done because with the blood of Oghama's angels flowing through their veins, these demons could travel the mists without forgetting their purpose. Mardero was the last and greatest of these demons.
Of course, this doesn't exactly fit into FfH history. In FfH, Erebus is not Oghma's Vault but a separate plane created by all the Gods acting together. The mist is not there. In this version, Merdero and his brethren were instead bread because their mortal blood allows them to act directly in creation without violating the compact.