Cossa? *Spoilers
What happened to Cossa after episode one? He was my favorite character.
He is elected Pope, but in later episodes I think the Pope appears, but it is not him.
As far as I know, they never really mentioned to what happened to him.
In real life, he was deposed from office. Remember when they mentioned that there was one Pope in Avignon, another Pope in Naples, and he would be the third that would try to unify the office? Long story short, the church unified behind someone else.
Wikipedia has a decent article: https://en.wikipedia.org/wiki/Antipope_John_XXIII