Illustration of a group Templars being burned at the stake.

Why are Fridays that fall on a month’s 13th day so fearful?

Some attribute the origins to the Code of Hammurabi, one of the world’s oldest legal documents, which may or may not have superstitiously omitted a 13th rule from its list. Others claim that the ancient Sumerians, who believed the number 12 to be a “perfect” number, considered the one that followed it decidedly non-perfect.

One of the most popular theories, however, links Friday the 13th with the fall of a fearsome group of legendary warriors—the Knights Templar.

READ MORE: The Knights Templar: The Rise and Spectacular Fall of God's Holy Warriors

Founded around 1118 as a monastic military order devoted to the protection of pilgrims traveling to the Holy Land following the Christian capture of Jerusalem during the First Crusade, the Knights Templar quickly became one of the richest and most influential groups of the Middle Ages, thanks to lavish donations from the crowned heads of Europe, eager to curry favor with the fierce Knights. By the turn of the 14th century, the Templars had established a system of castles, churches and banks throughout Western Europe. And it was this astonishing wealth that would lead to their downfall.

Illustration depicting the Knights Templar in battle, based on a fresco in the Chapel of the Templars in Cressac sur Charente, France. (Credit: DeAgostini/Getty Images)

For the Templars, that end began in the early morning hours of Friday, October 13, 1307.

A month earlier, secret documents had been sent by couriers throughout France. The papers included lurid details and whispers of black magic and scandalous sexual rituals. They were sent by King Philip IV of France, an avaricious monarch who in the preceding years had launched attacks on the Lombards (a powerful banking group) and France’s Jews (who he had expelled so he could confiscate their property for his depleted coffers).

In the days and weeks that followed that fateful Friday, more than 600 Templars were arrested, including Grand Master Jacques de Molay, and the Order’s treasurer. But while some of the highest-ranking members were caught up in Philip’s net, so too were hundreds of non-warriors; middle-aged men who managed the day-to-day banking and farming activities that kept the organization humming. The men were charged with a wide array of offenses including heresy, devil worship and spitting on the cross, homosexuality, fraud and financial corruption.

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The Templars were kept in isolation and fed meager rations that often amounted to just bread and water. Nearly all were brutally tortured. One common practice used by medieval inquisitors was the “strappdo,” in which the hands of the accused are tied behind their backs, and then suspended in the air by a rope around their wrists, intended to dislocate the shoulders. As Dan Jones notes in his book, The Templars: The Rise and Spectacular Fall of the Knights Templar, one of the accused’s hands were tied so tightly that blood pooled in his fingertips, and he was kept in a pit no wider than a single footstep. Many of the men were likely stretched on the infamous rack, or had their feet dipped in oil and held over a fire to burn. Given the extreme conditions, it’s not surprising that within weeks, hundreds of Templars confessed to false charges, including Jacques de Molay.

Portrait of Grand Master Jacques of Molay. (Credit: Stefano Bianchetti/Corbis via Getty Images)

Pope Clement V was horrified. Despite the fact that he’d been elected almost solely because of Philip’s influence, he feared crossing the extremely popular Templars. The Knight’s coerced “confessions,” however, forced his hands. Philip, who had anticipated Clement’s reaction, made sure the allegations against the Templars included detailed descriptions of their supposed heresy, counting on the gossipy, salacious accounts to carry much weight with the Church. Clement issued a papal bull ordering the Western kings to arrest Templars living in their lands. Few followed the papal request, but the fate of the French Templars had already been sealed. Their lands and money were confiscated and officially dispersed to another religious order, the Hospitallers (although greedy Philip did get his hands on some of the cash he’d coveted).

READ MORE: Top Templar Sites in Western Europe

Within weeks of their confessions, many of Templars recanted, and Clement shut down the inquisition trials in early 1308. The Templars lingered in their cells for two years before Philip had more than 50 of the them burned at the stake in 1310. Two years later, Clement formally dissolved the Order (though he did so without saying they’d been guilty as charged). In the wake of that dissolution, some Templars again confessed to gain their freedom, while others died in captivity.

In the spring of 1314, Grand Master Molay and several other Templars were burned at the stake in Paris, bringing an end to their remarkable era, and launching an even longer-lasting theory about the evil possibilities of Friday the 13th.