To which particular chapter are you referring to? I assume that you are talking about Chapter 19, the last chapter of Book Two of this excellent novel dealing with the intrusion of one culture into another. This chapter is of course when Okonkwo throws a banquet to celebrate the end of his seven years of exile, and towards the end, one of the oldest members of the umunna rises and gives a speech thanking Okonkwo for his hospitality, but more importantly, for the way that he is trying to maintain the tribal identity of the Igbo tribe. Note what he says:
"But I fear for you young people because you do not understand how strong is the bond of kinship. You do not know what it is to speak with one voice. And what is the result? An abominable religion has settled among you. A man can now leave his father and his brothers. he can curse the gods of his fathers and his ancestors, like a hunter'd dog that suddenly goes mad and turns on his master. I fear for you; I fear for the clan."
The ummuna ends by thanking Okonkwo for calling the tribe together in this way. Thus this speech foreshadows the destruction of the particular tribal identity that has kept the tribe so strong and united for such a long period of time. It likewise points towards the way that Christianity has already began to erode the principles and unity of the tribe, indicating the future problems it will cause later on.
i don't know exactly about the end how it is foreshadowing but i know that more than one action in the play was a foreshadow to what happens.. for example when Okonkwo is told that his action of beating his wife is something that would vanquish a whole clan! and another foreshadowing action is when he was told that having a hand in killing Ikemufena who used to call him father is an action that would vanquish and finish a whole tribe. those two main actions were considered a foreshadow for what happens at the end when the tribe doesn't actually exist in the same way again! and that the Missoinaries who came did actually vanquish the tribe and its traditions. hope my answer was usefull :D