According to legend, the Knights Templar was founded in 1118 to protect tourists headed for Jerusalem. There the small band of warrior monks established a headquarters. In due course, they somehow managed to gain possession of the Ark of the Covenant, which they hid for safekeeping.
Even though the Templars swore allegiance to the Pope, their beliefs were not exactly orthodox. They believed that Jesus Christ had conceived three children with Mary Magdalene, who moved to France after the crucifixion. Their progeny married into royalty; hence, there existed members of the French nobility who had divine blood in their veins. The Knights Templar eventually dedicated themselves to protecting the lives of these descendants of Jesus, the Merovingians.
But when they weren't busy doing that, they managed to amass a financial empire with vast holdings. The Templars created the first international banking system. Pilgrims heading off to the Holy Land would deposit money with a local branch, in exchange for a letter of credit, redeemable just about everywhere along their route.
By the dawn of the 14th century, the Knights Templar had become Europe's dominant religious order. They were more than 7,000 members strong. They held almost 900 castles. And they controlled unimaginable wealth in land, gold, and let's not forget that Ark of the Covenant.
In France, King Philip IV owed the Templars a lot of money, which he didn't really feel like repaying. So he hatched a scheme with Pope Clement V. In 1307, the Pope summoned Jacques de Molay, the group's Grand Master, to Paris. There they were supposed to enter negotiations on merging the Knights Hospitallers with de Molay's group. No such luck. Instead, he and his staff were arrested by King Philip and turned over to the Inquisition. Philip's men swept through the country, in just two days arresting 15,000 men associated with the accused.
Some of the Templars escaped to Portugal, where (for a few years) they enjoyed the protection of its king. Others were rumored to have fled to Switzerland and Scotland, but there is no documentation of this.
The Pope forcibly disbanded their organization in March 1312, announced in a Papal Bull accusing the Templars of having fallen "into the sin of impious apostasy, the abominable vice of idolatry, the deadly crime of the Sodomites, and various heresies." Plus, renouncing Jesus and spitting on the cross.
In March 1314, Jacques de Molay was sentenced to life imprisonment by the Inquisition. The sentence was announced during a public ceremony at Notre Dame cathedral. In exchange for his full confession, de Molay had received leniency. But before they could stop him, de Molay suddenly retracted his confession:
So, instead of life in prison, de Molay and his subordinate Geoffroy de Charnay were burned alive three days later. They were taken to the Isle of Javiaux in the Seine, not far from Notre Dame. There they were slow roasted over a pile of hot, smokeless charcoal. It took hours for them to die. Finally their ashes were collected for relics by Augustinian monks.
Eventually, the Church took possession of the assets belonging to the Templars. No word on whether the Ark of the Covenant was taken to Rome.
Today, the Knights Templar is the name of a branch of Freemasonry. It seems rather doubtful that the modern organization could ever conclusively trace its lineage back to their medieval namesake... but then again, everybody used to think the Coelacanth was extinct. So who knows?