Duergar: I'm shocked - a flavour change I really like!

[Najo]

Indeed.

Now why does any of that have to take my beloved industrial evil duergar away?

I mean, there's 100,000 different ways to do "scary devils, possession, corrupted magic, evil cults, underworld kingdom of evil." One of the ways to NOT do it, IMO, is to take over a race that already had a good thing going for it.

See, you keep the industrial evil duergar and aid into their mix corrupt cults, demonic possession and evil plots that tie in through specific warlords and high priests back to the hells. Then you got the best of both worlds and the ability to go deeper and further into the duergar with the ideals of evil other than they are really mean dwarves. That doesn't mean put horns on all of them, it means they embrace demonic power as a means for their leaders to use and abuse.