In the 13th century, there were remained parts of Italy which was attached to an old heresy known as Catharism. The parents of Peter of Verona were among the many adherents to this heresy, which denied the oneness of the Blessed Trinity among other errors.
Eventually Peter met St. Dominic who was the founder of the Order of Preachers (later called Dominicans), and joined his new religious order. Peter was such a brilliant preacher, but also he had a deep knowledge of the faith and was appointed as an “inquisitor” by the pope.
Peter preached throughout Italy, trying to root out some wayward beliefs and bring people back into the fold of the Catholic Church. His exact role as a defender of the faith attracted many enemies to him as well, who did not want him taking people away from Catharism.
According to the Catholic Encyclopedia, it narrates what happened to Peter next.
When returning from Como to Milan, Peter met a certain Carino who with some other [Cathars] had plotted to murder him.
The assassin hit him with an axe on the head with such violence, that the holy man fell half dead. As he rose to his knees he recited the first article of the Symbol of the Apostles which is the Apostle’s Creed, and as he offer his blood as a sacrifice to God he dipped his fingers in it and wrote these words on the ground: “Credo in Deum”. [I believe in one God] The murderer went further and pierced his heart.
After his death, his murderer renounced his heretical beliefs and joined the Dominicans in their mission and he led a life of holiness for the rest of his life.
St. Peter of Verona, sometimes recognized as Peter Martyr, became the first Dominican martyr to be canonized a saint.