In short, Ivan goes crazy in book 11. He starts talking to what he believes is the Devil every evening. The greatest part in this book though, is that we learn that Smerdyakov is actually the murderer. He admits to Ivan that he did it, but he expected that Ivan knew. He claimed that during Ivan and Smerdyakov's talk about shamming epileptic fits and rather Ivan should go to Chermasnya or Moscow, Ivan told Smerdyakov that Ivan wanted him to kill Fyodor Karamazov. Ivan didn't realize that he did this, but admits to it. Which is stupid. Then Smerdyakov gives a detailed account of how he killed Fyodor. Ivan goes home, and "talks to the devil" and then Alyosha comes and tells Ivan that Smerdyakov hanged himself.