I came to our group one evening with Green Ronin's Hammer and Helm. Two of my players are huge fans of dwarves, so when I mentioned the possiblity of doing an all dwarf camapaign they immediately liked it. The other two thought it would be interesting as well.
The city they are going to look for is the fabled Mar-Namor (It is mentioned in Hammer and Helm).
In that book it is said that the dwarves turned to the art of necromancy, they then created an army of undead to help defend against the mind flayer army. The dwarves that turned to the dark arts eventually betrayed the others and fled the city. The city of Mar-Namor then fell to the Mind Flayers.
I am going to alter this a bit, saying that the dwarven necromancers were a secret sect in the city. They were actually in league with the mind flayers, and as they fled took the three sacred stones created by the forge god and given to the dwarven race. The decendants of this sect will be what today is thought of as dark dwarves. Thinking either Druegor or Derro. I plan on making these dwarven necromancers worship Orcus either using the standard cleric, or the thautamerge (sp) from Green Ronin.