This is the second Council of Pisa question I've seen in three months; I wonder what drives fascination with this obscure event.

In any case, you can read my answer to the previous question here. In short, the Hispano-French camp, backing Benedict XIII seated in Avignon, went so far as to stage a counter-council in Perpignan (in the south of France). Not to be outdone, Germano-Italian camp, backing Gregory XII seated in Rome, also staged a counter-council in Aquileia (an ancient episcopal seat near Gregory's native Venice).

The German representatives, who arrived a month late, were met with hostility both inside and outside the council and eventually had to flee the city. The French representatives, who arrived three months late, were not any more welcome and also left before the council was concluded. Unencumbered by any opposition, the Council of Pisa declared both Benedict and Gregory heretical to the wide acclaim of all remaining participants, and after an eleven-day conclave elected a new pope: Pietro Filargo, a worldly man of Greco-Venetian origin. Bishop of Milan at the time of his election, Filargo had been one of the council's greatest initial sponsors and organizers.

Enthroned as Alexander V, the hope was that both Benedict and Gregory's supporters would abandon their respective camps and rally behind the new pope for the sake of the church's unity. Indeed, much of the council had revolved around the personal qualities of the two pretenders, and the feeling was that if only a less stubborn man were to lead the church the schism might somehow be brought to an end. The choice of Filargo, an ethnic Venetian who had studied in France and spent most of his career in Lombardy, was almost certainly made in the hopes of selecting a candidate who would appease as many camps as possible. Needless to say, he didn't.