kk, I see there's a lot of ideas running about with arguable creativity.
If there's so much original ideas, why not made something new from the old school ideas? Like say we want undead, but don't want that to be a copy of the other undead idea we see.
My suggestion is that to make a completely fictional cult that will use the undead (or something) because they believe immortality can be achieved. The followers of the cult wanted to keep their lives as they see many others expiring from death, and this is why they take a part of this bleak cult. Some wanted to bring back family, friends, and beloved ones. Others seek security from harm and shivered at the thought of their own end. Despite different backgrounds, the cultists mustered both skills in magic and technology (in the medical field and chemistry) to finally create the undead. The undead is not the cultists had in mind of immortality, but any success had them believed strongly in their efforts.
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Sorry for the detailed story, just my suggestion.
I thought about the undead using cold-base attacks and ain't the ice element already have that? Since we are talking about the undead, we can use 'death-based attacks' like plagues and rotting decay. That says dead to me. :idea: