Mutants....Ok, are we talking about mixed genes (Animal/human genes), or mutations in the form of unusual skills? If it's skills, go with darker stuff. You always hear about skills in fire or other elements. Maybe have darkness as an element and it covers shadows, death, dark things in general. Maybe there hasn't been someone like that in a while and the people who are like that die young because they go insane, or they feel guilty; maybe something like that. I hope this helps!