After the Israelites destroyed Jericho, killed almost everyone living there, and stole all their valuables, God got angry and decided to abandon his people and stop protecting them. Not because he didn’t approve of the massacre or anything, but just because one guy, Achan, had stolen some of the stolen goods, which God had intended to keep for himself for some reason. Shortly after that, God decided he didn’t mind his people keeping the plunder for themselves after all. But not before having Achan and his innocent children stoned and burned.
So that’s who Achan is, but who’s his father?
Achan is introduced as the son of Karmi, and the great-grandson of Zerah. But after that, the Bible keeps calling him the son of Zerah.