I think “most important event” is pretty subjective, but to me one of the most significant events is when Okonkwo kills Ezeudu’s son.
Okonkwo’s whole life is “dominated by fear, the fear of failure and weakness” (ch 2). Ezeudu is a well-respected elder.When Okonkwokills his son, it is an accident. His gun goes off during Ezeudu’s funeral. However, it is considered an affront to the Earth Goddess Ani, so Okonkwo is exiled for seven years. This event is important because Okonkwo has to leave to go to his mother’s family’s village. When he returns, his own village has changed quite a lot. It is becoming a colony due to the palm-oil trade.