In chapter 20 of Neil Gaiman's American Gods, Shadow returns to the apartment of Czernobog and the three sisters to make good on a wager made earlier in the story. Czernobog has held up his end, and now Shadow must hold up his, which means getting down on his knees, closing his eyes, and allowing Czernobog to smash in his head with a hammer.
When he enters the apartment, Shadow is told by Zorya Utrennyaya that a guest is expected and that he (Shadow) should come back the next day, when Czernobog will be gone. Exactly who the guest and why Czernobog would be gone were not obvious to me when I first read the story, but looking at it now, I'm quite sure that the guest is Bielebog, Czernobog's brother, who has been absent up to this point in the story but has been referred to by name and said to be Czernobog's exact opposite.
The names Czernobog and Bielebog are drawn from studies of early Slavic religion, where people apparently worshiped a dualistic god characterized by black and white, dark and light, and harshness and kindness. Czernobog is the dark side of the god: when he plays checkers, he always picks black; he enjoys killing things with his hammer; and so on. His brother Bielebog is the opposite. At this point in Gaiman's story, the seasons are changing from winter to spring, and that change supposedly marks the transition from the dark side of the divinity to the light side of divinity.
I used the words "apparently" and "supposedly" in that previous paragraph because there is disagreement among scholars of early Slavic religion as to whether or not Czernobog (sometimes spelled Chernobog) indeed had a opposite force, a brother of light to oppose his darkness.