Another video captures militants with the Islamic State of Iraq and Syria (ISIS) herding hundreds of boys and Iraqi soldiers down a highway to an unknown fate. “Repent,” ISIS told inhabitants of its newly conquered territory on Thursday. “But anyone who insists upon apostasy faces death.”

Death was everywhere in the sacked the city of Mosul, a strategically vital oil hub and Iraq’s largest northern city. One reporter said an Iraqi woman in Mosul claimed to have seen a “row of decapitated soldiers and policemen” on the street. Other reports spoke of “mass beheadings,” though The Washington Post was not able to confirm the tales.

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But the United Nations Human Rights chief, Navi Pillay, said the summary executions “may run into the hundreds” and that she was “extremely alarmed.”

The stories, the videos, the acts of unfathomable brutality have become a defining aspect of ISIS, which controls a nation-size tract of land and has now pushed Iraq to the precipice of dissolution. Its adherents kill with such abandon that even the leader of al-Qaeda has disavowed them. “Clearly, [leader Ayman] al-Zawahiri believes that ISIS is a liability to the al-Qaeda brand,” Aaron Zelin, who analyzes jihadist movements for the Washington Institute for Near East Policy, told The Washington Post’s Liz Sly earlier this year.

But in terms of impact, the acts of terror have been wildly successful. From beheadings to summary executions to amputations to crucifixions, the terrorist group has become the most feared organization in the Middle East. That fear, evidenced in fleeing Iraqi soldiers and 500,000 Mosul residents, has played a vital role in the group’s march toward Baghdad. In many cases, police and soldiers literally ran, shedding their uniforms as they went, abandoning large caches of weapons.

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“We can’t beat them,” the Sydney Morning Herald quoted one soldier as saying. “We can’t.”

The commitment to shocking violence is at the heart of both ISIS’s recruitment and appeal. To radicalized Islamists across the world, there’s something enticing in ISIS. It has attracted at least 12,000 fighters — 3,000 from the West — since its inception several years ago.

“Allahu Akbar,” wrote one British jihadist in an Instagram post that showed a militant among several severed heads and a fake skeleton. “Our Brother Abu B of ISIS posing with his two trophies after the operation yesterday. The skeleton is not real :)”

“My first time!” the Brit says beside another image of a hand covered in blood.

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It’s difficult to say what spawned such fealty to violence. “There is absolutely nothing in Islam that justifies cutting off a person’s head,” a professor at the Center for Muslim-Christian Understanding at Georgetown University once told Newsday.

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But just as the Bible discusses the beheading of John the Baptist, so does the Koran talk of beheadings. “When you meet the unbelievers in the battlefield, strike off their heads until you have crushed them completely; then bind the prisoners tightly,” Sura Chapter 47 says.

The act, despite its religious underpinnings, can be manipulated into terrorism, Timothy Furnish wrote in a 2005 Middle East Quarterly article. “The purpose of terrorism is to strike fear into the hearts of opponents in order to win political concession,” he wrote.

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Islamist terrorism, he said, has gone through several phases: hijacked airlines in the 1970s and 1980s, car and suicide bombs in the 1980s and 1990s. But the “shock value” of each inevitably wore off, giving way to something new “to maximize shock and press reaction upon which they thrive,” he wrote. “What once garnered days of commentary now generates only hours. Decapitation has become the latest fashion. In many ways, it sends terrorism back to the future. Unlike hijackings and car bombs, ritual beheading has a long precedent in Islamic theology and history.”

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But “increasingly,” Furnish wrote, “Islamist groups conflate ‘unbelievers,’ ‘combatants,’ and prisoners of war, which, coupled with their claim to Islamic legitimacy, provides them with a license to decapitate.”

This license is one of ISIS’s most salient traits. Twitter is awash with images of its decapitations and worse. The result: Fear has become a potent ISIS weapon, according to this Amnesty International report called “Rule of Fear.”

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“I didn’t want to be taken by them … so I started running,” one former ISIS prisoner told the human rights group. “They ran after me, all masked, and captured me. I started shouting loudly to get the attention of the crowd of people: ‘What have I done? What have I done?’ I could see people looking at me, but no one said a word. They were all killed by fear.”

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Indeed, in another recently-released video reported by the Associated Press, ISIS fighters capture a tribal militia commander along with his two sons. The prisoners are forced to dig their own graves. “I advise whoever is with the Sahwa to repent and quit,” the commander says into the camera. “Here I am digging my grave with my own hands…. They can get to anyone.”

Then the jihadists slit their throats.