The Missing Dwarves of the Khaziram Mountians

Five hundred years ago, the dwarven clans of the Khaziram Mountains disappeared. No one knows where they went or why they went missing.

Summary

500 hundred years ago the dwarven clans of the Khaziram Mountains disappeared from their holdfasts and mines and while several holdfasts remain occupied, most notably Nirnlodir, most have remained empty since then. The dwarves that remained are the ones that had close ties to the human and elven communities nearby, but those dwarves remain silent on the question of what has become of their brethren.
Several tales have arisen amongst the human communities that border the former dwarven territory. The most common is that the dwarves in the emptied holdfasts dug too deep into the earth and released an evil that overpowered them. Some say that the dwarves mined into deep caverns full of the Fallen, hiding since the Great Incursion, and that they were quickly overcome and turned into demons themselves and that is where Tieflings come from. Others say the clans found a great treasure, perhaps a vein of mithral, and their own greed caused them to fight against one and other until they all lay dead, and their haunted, greedfilled spirits still roam their strongholds and mineshafts.
The elves of the Sylint Forest have explored most of the lands around the holdfasts in the southern Khaziram Mountains, and they claim that the holdfasts have an unnatural feel to them, and they avoid them saying whether living or dead the dwarven holdfasts surely are cursed.
Occasionally the old holdfasts are explored by adventurers looking for long forgotten treasures left behind, but few return, and those that do, stare into the fires and claim they did not see anything of note...
These stories are told around campfires, and kitchen tables, and to young children as they are tucked into bed. The tales always end with the warning to stay out of those haunted mountains, or they might disappear like the dwarves and haunt the halls of the Khaziram forever.