Appendix 2, The Corruption of the Duergar
When Reorx created the Dwarves, he instilled in them a love for working with stone, gems, and any other raw materials. Ultimately, that would be part of some Dwarves' downfall, and would give birth to the Duergar race.
Remoah was once a peaceful Immortal. However after watching his older rival Reorx create the well-known and industrious race of Dwarves, he flew into a jealous rage and has not recovered since.
Thousands of years ago, Remoah approached a group of Dwarves who were on their way back from a mining expedition. He offered a trade to them. He would give them eternal life if only they would give him their hoard of gems that they had just mined. The group, greedy to the bone, had accumulated approximately one ton of precious gems, a good haul for back then, and something that could not possible happen during the present time period.
Driven by malice, Remoah offered the Dwarves an impossible deal. He was a powerful Immortal, however he did not have the power over being an Immortal versus being a Mortal.
Seeing a good deal, the Dwarves readily agreed to the deal. However, Remoah took the hoard of gems and vanished abruptly, heading back to his home in the Chaos Plane.
Disgruntled, the jilted Dwarves wanted their prize. Posing as diligent priests for Reorx, they approached their Patron to ask for guidance and help. Reorx, being the aware Immortal that he is, knew exactly what was going on. He refused the Dwarves' help and told them that being gullible was not a Dwarvenly trait.
Well, the jilted Dwarves did not like this, so they sought out help in other means. They approached a half-mad Dwarf by the name of Rhettu to help them. Rhettu was rumored to be a priest of Remoah, and could do extraordinary miracles.
Rhettu knew what was going on, as Remoah was indeed his Patron. Remoah instructed Rhettu to lead them into the dark, as it were, to try and corrupt those Dwarves into doing atrocities onto which Reorx would even turn his back upon.
Well, Remoah's plan worked. Those Dwarves did whatever it took, killed whoever they had to, and stole whatever they needed to. Eventually, their murderous and treacherous path led them to deep into the world. They ended up far below the surface of Kalin. Coming to a deep area south of the main Drow city, the Dwarves came upon a dark altar.
Coming to them into a dream, the Dwarves prayed to the dark altar and asked for a plan of action. What came to them, well, perplexed them.
They were told they had to stay down in the UnderDark. However, their prayers would be answered and they would be blessed with gems. Gems enough to fill all of their remaining days, which would be great. Agreeing, the Dwarves believed they had a good deal.
That dark altar was indeed an altar to the now-known Corruptor, Remoah. Remoah had tricked those Dwarves into denouncing their benevolent Patron, Reorx. That group of Dwarves was destined to live the rest of their days down in the darkness of the UnderDark, scratching out an existence.
Eventually the surface Dwarves heard of this, and were appalled. It came to a time where any surface Dwarf would attack any UnderDark Dwarf (now called the Duergar) on sight.
After time, the Duergar built cities, becoming a rival to the Drow as masters of the UnderDark.