The Strangers are a non-humanoid hive-minded parasitic species with an advanced technology and extensive psionic powers. They are know to have a natural aversion for water and direct sunlight.
They visited Earth in the 1940ies, when their collective mind was in its terminal state of decay and abducted some human specimens to experiment with the hope of undoing their doom by understanding humankind's individuality.
They possessed some humans whose body they used as vehicles and populated a huge space habitat losely modeled on a north american city of the era with a large population on whose memory they started experimenting.
Most of them are believed to have died when a human subject developed psionic powers superior to their own and took control of the city, releasing large bodies of water an turning the habitat so that it stood exposed to sunlight.
Source: Alex Proyas,
Dark City