Complex sentence: He was offered the choice of resigning his see, in which case he would become emeritus bishop of Evreux, or being removed from the see, in which case he would simply be the ex-bishop of Evreux, though retaining his titular see of Partenia.
Split into simpler sentences: He was offered the choice of resigning his see and become emeritus bishop of Evreux, or being removed from the see. In that case, he would simply be the ex-bishop of Evreux, though retaining his titular see of Partenia.