It gets its name from the town of Tridam, where a local baron had thrown some brigands into a dungeon. The bandits' friends (among them the half-elf Civril) held a ferry full of pilgrims as hostages, demanding their friends' freedom in exchange for the people's lives.
The baron refused, and the bandits began throwing the pilgrims overboard. Dozen were killed in this manner before the baron changed his mind and freed the brigands.
Not everyone agreed with his choice; solutions to the situation that would not have allowed the bandits to go free were offered, along with suggestion for the baron to be exiled or executed. The baron argued that he chose the lesser evil by letting the men go free, and thus saving the lives of over twenty-five people.
This happened on the Feast of Nis, about three years before the short story The Lesser Evil, in which Renfri and her band (including aforementioned Civril) attempted to do the same with the people in Blaviken market and the wizard Stregobor. What transpires that day leads Geralt of Rivia to be known as the Butcher of Blaviken.