The Western Schism

Months after the papacy's return to Rome, a terrified conclave elected Urban VI under a mob's pressure; when his behavior curdled into violent paranoia, the same cardinals declared the election void and chose a rival, who went back to Avignon. Europe split down the middle, saint against saint: Catherine of Siena for Rome, Vincent Ferrer for Avignon. A council at Pisa, trying to fix it, produced a third claimant.

Only this timeline's Roman line carries the ordinals here, but for four decades no one alive could be certain. The Council of Constance finally cleared the board and elected Martin V in 1417, at the price of a new idea with a long future: that a general council might stand above a pope. The rest of the era is the popes wrestling that idea back down.