In the book Things Fall Apart, in Chapter 8, what warning does Obierika give Okonkwo?
Obierika tells Okonkwo that nothing good will come from his participation in the killing of his adopted son. He warns,
"What you have done will not please the Earth. It is the kind of action for which the goddess wipes out whole families."
In Chapter 7, the Oracles had decreed that Ikemefuna, Okonkwo's adopted son, should be put to death. Okonkwo is advised not to take part in the ritual killing, but he goes along anyway. When the first blow is struck, Ikemefuna runs, dying, to his father, who finishes the job with his machete. Okonkwo is haunted for days by what he has done, and can neither eat nor sleep.
In Chapter 8, Okonkwo goes to visit his friend Obierika, who tells him that he should not have taken part in the sacrifice of his son. Obierika says that had it been his son who had been involved, he would not have disputed what the Oracle had decreed, but neither would he have participated in the killing. Obierika would have simply stayed home, and let others do the dirty work. He points that by acting in this manner, he would not have been questioning or defying the Oracle's authority, since he had not been specifically instructed to carry out the ritual himself. The Oracle's mandate thus would have been fulfilled, and Obierika would have been spared the guilt and stigma of killing his own son.
In Chapter 7, the Oracle of the Hills and Caves has decided that Ikemefuna must die. When Ezeudu tells Okonkwo of this decision, he tells him not to participate in the child's killing because Ikemefuna regards Okonkwo like a father. Nevertheless, Okonkwo not only participates, but also delivers the final blow with his machete.
In Chapter 8, Obierika tells Okonkwo that he should not have participated in the killing of Ikemefuna. He says,
What you have done will not please the Earth. It is the kind of action for which the goddess wipes out whole families" (67).
If you read on, you will see that the warning was foreshadowing the consequences of Okonkwo having disregarded Ezeudu's advice.