I was going to save this for some later time, but give recent events it seems that it may be of more use now. I hope that my guess is wrong, but if it isn't waiting will only hurt in the long run.
In my fandom, the darkness takes physical form. We call them the 'Heartless' - perhaps a misnomer since the first were born from people's hearts. They are mindless creatures of the dark driven on by nothing more than their unending hunger for living hearts, by it animal or human; left unchecked they can and will seek out the very heart of a world and consume that as well, drowning the world in darkness. In essence, killing that world.
In addition, they are capable of traveling through the same darkness that first created them. As a result, they are capable of spreading almost remarkably fast, should they wish. I am unsure of whether they can cross the boundaries that separate fandoms from each other, but within my own fandom there are very few worlds that don't see at least some form of Heartless these days.
Also, I should note that except for a very few exceptions, someone whose heart is stolen by a Heartless will become a Heartless themselves. The most common type of natural Heartless looks like a shadow with yellow eyes, but it's not unknown for them to take
other forms as well. As for the unnatural variety, well, I'd hope for your sake that no one's figured out how to create them.