On April 26, 1478 in the cathedral of Florence during the mass, the Pazzi family and their cronies attacked Lorenzo and Giuliano de Medici. Lorenzo was wounded but Giuliano was killed. It had been a plot very well prepared; they tried to kill them both at dinner, but Giuliano did not attend. So the plans were changed, and they attacked them in the cathedral.
It is said that the plot was approved by Pope Sixtus IV and the Bishop of Pisa, who was not named Archbishop by Lorenzo in Florence.
When the Pazzi and his assistants were arrested, the revenge was huge: the bodies of the Pazzi were tied to horses and dragged through the city, hanged from the windows of the signoria and other buildings in the square, etc.
Bertoldo di Giovanni created a commemorative medal of the plot, today in the Bargello.
Machiavelli in his book “Istorie Fiorentine” includes a book named “The congiura dei Pazzi e l’età di Lorenzo il Magnifico”.
It was not until more than 500 years later that the true architect of the plot was discovered – none other than Federico da Montefeltro: http://elpais.com/diario/2004/02/20/ultima/1077231601_850215.html
I said that this it’s just a summary…