Galeazzo Maria Sforza (1444 – 1476) was the duke of Milan, husband to Bona of Savoy and partner of Lucrezia Landriani. Through the relationship with the latter, Galeazzo was the father of Caterina Sforza, who would later become the countess of Forlì and Imola.
In 1476, the cities of Milan and Florence declared an alliance, thereby strengthening each other culturally, economically, and militarily. The alliance had a negative impact on the Templars as they were conspiring to take the city of Florence. Aware of the fact that Sforza's death would weaken, at least temporarily, Lorenzo de' Medici's hold on his city, they brought up a plan to murder him.
During the same time in Florence, Giovanni Auditore da Firenze and Lorenzo de' Medici were hearing rumors about an upheaval in Milan; Lorenzo then sent the Assassin to investigate. Not long after, one night in a Florentine alley, Giovanni attacked a group of Templars led by Rodrigo Borgia. While Borgia inevitably fled, Giovanni kept one of the Templars alive for information. As a result, Lorenzo's men tortured him while Uberto Alberti conducted the interrogation, and under the pressure of pain, the Templar finally revealed their plan.
On the 26th of December, Saint Stephen's Day, Sforza would go to the Basilica di Santo Stefano Maggiore to attend the mass. Three Templars, Carlo Visconti, Gerolamo Olgiati, and Giovanni Andrea Lampugnani were chosen to murder the duke. On the day, Giovanni Auditore traveled to Milan and entered the church during Mass where he then saw Lampugnani kneeling before Sforza and kissing his hand.
Realizing the danger, Giovanni tried to cross the crowd to reach them but he was too late to save the duke, as Lampugnani suddenly rose and stabbed Galeazzo through his chest. The two others then joined Lampugnani as a dozen of their armed allies fought Sforza's bodyguards and Giovanni; Visconti and Olgiati, having stabbed their enemy to death, then fled. Lampugnani soon began to run as Sforza's Moorish bodyguard came after him, but ended up trapped in a cul de sac and killed, despite Giovanni's attempts to dissuade the bodyguard. Subsequently, Giovanni had failed to save the duke, thus the plan proved successful.