wallacer:
If you don’t want to spend a lot of time converting them, then Chaotic shields and a suitably grim paint job should be more than enough to make them look evil.
If you want to go the whole hog then something along these lines should be manageable without a vast amount of effort:
https://discourse.chaos-dwarfs.com/t/5743
Have a look through the Conversions area and see what takes your fancy.