Well, here's how the darkness works:
Those who are infected/cursed are stuck in an internal war with the dark itself. Two things can happen: either you give into it's influence, or you die after weeks or even months of agony. Option one can be forced onto victims by the use of a ritual ((which cults and followers of the darkness try to use)) Once you have given into the influence you become stronger and more powerful, but your mind also becomes corrupted, your internal self becoming bound in chains while this new influence takes over and causes destruction. Nothing good can come from this evil thing, which is why it is the target and antagonist of this story