The Lonely Ones are a race of beings from a mirror universe. Literally. The story goes that if you look in a mirror, itss really another universe. That universe is similar to ours, except for one thing: it is infested with LOs. They hunt and kill humans there, who are fighting for survival.
The beings are all black, and can distort into any shape, as long as they stay somewhat humanoid. All mirrors are portals, and the parallel you is pushing you back oit to avoid the terror.