Zorya Polunochnaya is describing the doomsday hound, Simgarl.
In Slavic myth, Simgarl is chained to the north star Polaris. If he breaks free he will devour the constellation 'Ursa Minor', destroying the universe in the process. 'If it escapes, it will eat the whole of everything,' Zorya Polunochynaya tells Shadow. This is why she and her sisters sleep in shifts, watching the skies to see that the hound does not break free.