I'd keep the necrons of warhammer 40K in mind combined with that then.
Considering you just want to use skeletons there isn't that much we need to do on graphics though.. just take the skeleton sprite and give them an ulmish weapon.. and I think I even have the seperate weapons the ulmish sprites have. For Necrons you could probably just colorify a skeleton to grey/metallic.
When I'm done with the things I'm doing now I might be interested to start on this if the nation can be fleshed out a bit. I need a bit more details on what troops there would need to be etc etc.. if we just make them undead ulmsmen it might not be that interesting. we could go for the huge piramid ships the necrons have... huge expensive ships. and then next to that undead and robots... it does have some appeal.
But then you'd probably end up coping the death domain from ermor too... sinec that would be very thematic... do we want ermor like that in space?
It would make for another "different" nation though (even though we already know how it plays

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