What is the notion of the ogbanje which is treated at length in chapter 9 of "Things Fall Apart"?
Ogbanje is what the Ibo believe to be a wicked child. This child is born, then dies in infancy, and then enters his mother's womb to be born again. This belief caused dead babies to be mutilated to try and stop the wicked child from returning. Okonkwo's wife Ekwefi lost nine of her children and the medicine man told Okonkwo it was because of an ogbanje. Finally Ezinma is born to Ekwefi, and Ezinma lives although she goes through periods of illness. This story helps to reveal some of the tribal culture and belief. It also shows us a very human side to Okonkwo and Ezinma as they struggle with the loss of so many children and Ekwefi's illnesses.
The beliefs of the Ibos are also well illustrated in the novel. The superstitious beliefs held by the Ibo are not neglected in Achebe's narrative. The concepts of ogbanje  and iyi-uwa are given attention in the ninth chapter.
"The medicine man then ordered that there will be no mourning for the deadchild. He brought out a sharp razor from the goatskin bag slung from his leftshoulder and began to mutilate the child. Then he took it away to burry inthe evil forest, holding it by the ankle and dragging it on the ground behindhim. After such treatment it would think twice before coming again, unlesswas one of the stubborn ones who returned the stamp of their mutilation."(Achebe.p:55)
This comes in the story of how Okonkwo sought the help of a medicine man to