Duergar Solution
by Steven B. WilsonI was recently adapting a 4E adventure (H2 - Thunderspire Labyrinth) to use in Mystara and some of the main antagonists are duergar (Underdark Dwarves). I read some other posts discussing on how to adapt the Duergar to Mystara. Well, here is my solution.
Since I place the adventure on the border between Karameikos and Five Shires, I remembered the dwarven history of the Shires and when looking. Here was the genesis of the idea from GAZ 8:
"Loktal [Ironshield] found he had fewer than four hundred dwarves left. In the morning he quietly turned and marched north again, abandoning perhaps a thousand dwarves scattered across the mines and harbours of the land. Loktal was never heard from again in the land of the hin." ~ pg. 5
Those 400 dwarves led by Loktal disappeared into a massive cave system (about 912 BC) under the Black Spire mountains. Under the harsh guidance of Loktal, they came to believe that Kagyar had abandoned them so they turned to the Entropic Immortals in their anger. A bargain was struck and their fate sealed. The dwarves fell under the influence of Atzanteotl, who took great pleasure in corrupting and transforming the wayward dwarves. They endured uncounted years as thralls as they built their own city far beneath the surface of the world, nurturing their hatred for the world above. The darkest and vilest rumours abound that they strengthened their bloodline by breeding with lesser fiends - a rumour that perhaps has some glimmer of truth when their modern-day abilities (beard quills, strong resistance to fire and poison, etc.) are taken into account.
All that being said, the Duergar are still a fairly small group hidden deep beneath the Five Shires, but can be encountered anywhere due to their wide wanderings in the subterranean tunnels (did the name Shadowdeep ever catch on?).
Difference between the Modrigswergs and the Duergar: Perhaps not much, really. More a matter of location. But there is the fact of the Modrigswerg being presented as "normal" dwarves while the Duergar have more special abilities...of course, that is probably due to the difference in game mechanics. In my campaign, I'll likely have them essentially the same but with different histories - Atzanteotl is on the lookout for any clan of outcast dwarves to add to his corruption file. This can account for the social differences between the two clans while maintaining a sense of consistency (if Entropic Immortals can ever be thought of as "consistent").