The Darkness is a mysterious ongoing cosmological phenomenon plaguing the world during the events of The Banner Saga.
First mentioned by the Serpent to Juno in Chapter 5, it soon becomes evident that this Darkness is most likely the reason that drives the massive migration of the Dredge from the northern regions of Valkajokull to the south/south-west.
As of the end of the second part of The Banner Saga little is known about the Darkness. Theories suggest that it might be some 'reaction' of the world meant to balance the perpetual daylight caused by the other mysterious cosmological phenomenon: the stopping of the Sun in the sky...
In the third part, it is revealed that it was released on to the world as a consequence of Eyvind utilisting the Dark Sun giving light to the underground habitat of the Dredge, in order to ressurect Juno.