Antipope John XXIII

Cardinal Baldassare Cossa was one of the seven cardinalss who, in May of 1408, deserted Pope Gregory XII, and, with those belonging to the obedience of Antipope Benedict XIII, convened the Council of Pisa, of which Cossa became the leader.
They elected Pope Alexander V in 1409. Cossa succeeded him a year later.

Edward Gibbon asserts in The History of the Decline and Fall of the Roman Empire that John XXIII was charged with piracy, murder, rape, sodomy, and incest... with the more serious charges being suppressed. Of course, this needs to be viewed in the light of the political situation of the time, as the charges were likely trumped up; note the similarity to the charges against the Knights Templar.

He should not be confused with Pope John XXIII of the 20th century. The fact that for more than 500 years there was no Pope named John (vs. 23 in 1400 years) is probably due to the controversial figure this (anti)Pope represented. The decision of the 20th century Pope John XXIII not to be named John XXIV as might be expected serves as a confirmation of the antipope status of this first John XXIII. It should be noted, however, that the numbering of the Popes called John is debatable; for example, Gibbon refers to this Pope John as John XXII.

The North American student fraternity Kappa Sigma claims that its origins are from Cossa's governorship of Bologna, Italy, where his campaign of assault and robbery against the students of the University of Bologna led to a secret organization under the teacher Manuel Chrysoloras for mutual protection against Cossa's brigands.