The fourth column in mercenary towns.

Actually, there's no need to come up with anything new. If you remember the old lore, there were light and dark dwarves. Adding a dark dwarf faction to the third (or fourth) column would fit in quite well.
By the way, not only dwarves, but also the Inferno faction is somewhat lacking in new creatures. I've found a few quite good options, but there's not enough variety.