Describe the outcast in chapter 17 of Things Fall Apart.
I think you might be referring to chapter 18 where the outcasts of the tribe are described - the outcasts who are being accepted by the missionaries and their church. In chapter 17, the only outcast mentioned is a young woman who joins because she keeps giving birth to twins and the twins are killed.
In 18, the outcasts or osu, see that the church has accepted twins, so they go there too. These men were not allowed to participate in any of the tribal rituals, and they lived in a special place in the village alone. They were not allowed to ever trim their hair or wash it; not allowed to marry. And when they died, they were buried in the Evil Forest.
These men are significant because no one believed that they could be accepted with repercussion from the gods, even the new converts, but the men shave their heads, join the church, and nothing bad ever happens to them. The church gains strength as a result, and Okonkwo loses, because he wants the tribe to attack and kill the missionaries, but they choose not to. The white man's ways begin to subvert the tribal beliefs.