14 January 1208
This might be the costliest murder in history. In the early 1200s, the Cathar Christian heresy flourished in the south of France. The area back then was virtually independent from the French crown, and had a much more cosmopolitan culture than most of Europe (Muslim Spain, with its relative tolerance of Jews and science, was not far away). The Cathars were Christians who denied the divinity of Christ, which didn't make the Papacy happy. They sent a legate, Castelnau, to Toulouse to threaten the count, Raymond, with excommunication if he didn't get his people in line. Raymond backed down, but on his way back to Rome, Castelnau was murdered by a knight in the count's service. It may or may not have been on Raymond's orders (probably not - Raymond knew what such a murder would bring down on him). Pope Innocent III, perhaps the most powerful pope in history, was outraged, and declared a crusade against the heretics. Simon de Montfort, a minor French noble whose son would play a huge role in English history, was put in charge of the crusade. It is from this crusade that we get the famous (and apocryphal) line: "Kill them all, let God sort them out" (it deals with the siege of Beziers and how to tell Cathars from "good" Catholics, but was probably never actually uttered). Montfort and his successors ravaged the Languedoc and brought it more firmly under the suzerainty of the French crown - and they killed a lot of heretics as well. It wasn't until 1255 that the last Cathar stronghold was taken, long after Montfort has been killed in a siege ... ironically, of Toulouse. In 1231, the new pope, Gregory IX (oh, I'm so ashamed!), fully aware of the dangers of heresy, launched the Inquisition. Nice guy. Good legacy.
I find the Albigensian Crusade fascinating. I find 13th-century history fascinating, actually. Unfortunately, a lot of it has been tied up into the Templars and the Holy Grail, so there's a lot of false information out there. But there are a lot of neat books, too. It's one of those things that not a lot of people know about but which really changed the course of history. Very interesting. And tragic.