Not one institution but several: the medieval Inquisition that began around 1184 against the Cathars; the Spanish Inquisition founded in 1478 and controlled by the crown; the Roman Inquisition of 1542. All used secret accusations, torture, and the threat of burning to police what people believed. The Spanish tribunal's reputation as a machine of mass slaughter became a byword in Protestant Europe. Its own records, when historians examined them, told a different story: somewhere between 3,000 and 5,000 executions across 350 years — a real engine of fear, but far smaller than the legend.
For three centuries the Spanish Inquisition was remembered as a machine of mass slaughter, its dead counted in the hundreds of thousands or even millions. The strange thing is that the Inquisition itself kept the receipts. Its clerks recorded every accusation, every interrogation, every sentence, and stored the paper in their archives. When modern historians finally counted those records, the monstrous figure collapsed: the Spanish tribunal probably put to death somewhere between three and five thousand people across roughly 350 years, out of perhaps 150,000 it tried. That is a fraction of the legend. It is also thousands of human beings burned or strangled for what they believed, by courts that used secret accusations and torture, and that hounded converts from Judaism and Islam suspected of secretly keeping their old faith. The terror was real. The legend was larger.