The Council of Pisa opens for the purpose of ending the Great Schism of the West (1378-1417). Cardinals from both the Roman and Avignon factions meet. They do this without any papal approval because they think that an ecumenical council had authority over popes, something the popes themselves dispute.
The Council of Pisa will depose both the Roman pope (Gregory XII) and the Avignon pope (Benedict XIII) then elected Peter of Candia as a new, replacement pope. Peter will take the name Alexander V. Because neither of the two deposed popes approved of the Council, their supporters will refuse to acknowledge the new election. So not only is the Great Schism of the West not ended, but it has gotten worse because now there are three rival popes instead of two! The conflict will not be resolved until the Council of Constance (1414-1418).