What the medicine man means when he says Ekwefi's child is an ogbanje is that C : it is an evil spirit that reincarnates in her womb.
Things Fall Apart is a novel written by Chinua Achebe and published in 1958. It focuses on the lives of the people living in Nigeria during the pre-colonial era. Okonkwo is the main character of the story. His second wife is Ekwefi, who suffers the death of nine of her children during their early stage of development. When Okonkwo asks the medicine man the reason why she is losing her children, the man tells him that they are being tormented by ogbanje. Ogbanje is a term used in Odinani to refer to an evil spirit that is responsible for causing a family's misfortune.