Before the apocalypse, humans started to splice themselves with machines, this increased the capabilities of the human race to almost god-like perfection. But in only a short 10 years after the splicing that the human brain couldn't cope with these augmentations. They went insane, and viewed all un-spliced creatures as competition in survival, and so would sadistically torture and kill them. They're almost impossible to kill, but there are very few of them. They are a constant fear in the minds of all sentient earthly beings.