The Strangers from Dark City are an ancient, dying race.
They have a nasty habit of kidnapping humans and experimenting with them.
They want to find out what make humans tick believing that the secret can save their dying race.
They play with human memories changing peoples' personalities from time to time to see what happens. They use the bodies of dead people as vessels to move around.
They are unique among the parasitic types of aliens in that they seem to have unfathomable mental powers. They are telepathically connected to each other and can also make people sleep or, in the process they call tuning, reshape the city in a few minutes.
They have aversion to water and light so if you mix these ingredients the nightmare starts to fade.