Chapter 11 opens with a hyperbolic statement, intended to establish mood:
A palm-oil lamp gave out yellowish light. Without it, it would have been impossible to eat; one could not have known where one's mouth was in the darkness of that night.
This darkness may be total—it may be near-impossible to see without light—but it is extremely unlikely that a person would not be able to find one of their own body parts in the darkness. The statement that "one could not have known where one's mouth was in the darkness of that night" serves a figurative purpose rather than a literal one. Akin to the phrase, "it was a dark and stormy night," this instance of hyperbole sets the mood in Chapter 11, indicating that difficult and perhaps terrifying things will occur.
The above hyperbolic statement could also be considered foreshadowing. Later on in Chapter 11, this dark mood comes to fruition: Chielo, the priestess of Agbala, arrives in the middle of the night demanding to take Ezinma to visit her god. Terrified that she will lose her only daughter, Ekwefi follows Chielo on foot, tracking Ezinma and the priestess through the darkness. Ekwefi and Okonkwo are perpetually in fear for Ezinma's life; these circumstances only exacerbate their worry.