Many monarchs throughout history have killed family members. England’s Henry VIII, for example, beheaded two wives and several cousins.

Cleopatra engineered the murder of two siblings (one of whom was also her husband). And Atahualpa, the last Inca emperor, ordered the execution of his half-brother from a Spanish prison.

But even those royals might have been aghast at the actions of Russian czar Peter the Great, who in 1718 had his eldest son tortured to death for allegedly conspiring against him.

Peter I, better known as Peter the Great, is generally credited with bringing Russia into the modern age. During his time as czar, from 1682 until his death in 1725, he implemented a variety of reforms that included revamping the Russian calendar and alphabet and reducing the Orthodox Church’s autonomy. Peter even instituted a tax on beards as part of his efforts to make Russians look and act more like Western Europeans.

At the same time, Peter built Russia’s first real navy, updated the army and won a series of military victories. On land conquered from Sweden, his main antagonist, he established the city of St. Petersburg and then moved the capital there from Moscow. “Peter ended up…rebelling almost completely against [traditionalist] Muscovite political culture,” says Jonathan Daly, a professor of Russian history at the University of Illinois at Chicago.

Much to Peter’s chagrin, however, his eldest son and heir apparent, the tsarevich Alexei, grew up thinking differently. Alexei’s mother, Eudoxia, was both pious and conservative, and her marriage to Peter proved rocky. In 1698, when Alexei was eight years old, Peter left her and forced her into a convent. From then on, Alexei was raised largely by his aunts, though he also received Enlightenment-era tutoring in foreign languages and mathematics and studied abroad in present-day Germany.

Like many children of European monarchs, Alexei didn’t often see his father, who spent much of his reign away fighting the Ottomans and Swedes and traveling around Western Europe. Instead, Alexei was surrounded by a Moscow entourage that believed in “less westernization and a bigger role for the Orthodox Church and the aristocracy,” explains Paul Bushkovitch, a history professor at Yale University and author of Peter the Great: The Struggle for Power, 1671-1725.

As a teenager, Alexei was put to work “essentially in the logistics department of Peter’s army,” Bushkovitch says, where his tasks included “collecting food and recruits and sending them to the appropriate places.” He was also nominally charged with the defense of Moscow during Sweden’s failed invasion of Russia in 1708. “We don’t have from those years the sense that there is a problem” between Alexei and Peter, Bushkovitch says.

That began to change around 1711, when Peter married the tsarevich off to a German princess named Charlotte. Though relatively pleased with her husband at first, Charlotte soon found herself lonely and isolated, complaining in letters of Alexei’s emotional unavailability and excessive drinking. She died of postpartum complications following the birth of their second child in 1715, by which time Alexei had begun an extramarital affair with a serf, Afrosina Fedorova.

Meanwhile, still sore over his mother’s banishment, Alexei did not attend the 1712 wedding of Peter and his second wife, Catherine. Bookish, timid and far less physically imposing than Peter, the tsarevich constantly complained of ill health and is said to have once purposely injured his hand rather than submit to one of his father’s demands.

The father-son relationship cracked for good in October 1715, when Peter penned Alexei a letter bemoaning his lack of military prowess and threatening to deprive him “of the succession as one may cut off a useless member.” Peter added that he would rather pass the crown “to a worthy stranger than to my own unworthy son.”

In reprimanding Alexei, Peter apparently hoped to scare him straight. But the terrified tsarevich instead volunteered to relinquish his claim to the throne, saying that he felt unfit for service and that the czar should be “a more vigorous man than I am.”

Despite Alexei’s assurances that he wanted nothing to do with government, Peter worried that his opponents would rally around his son. After all, as Daly points out, “Alexei was allied with many of the interests and forces within society and the political elite who opposed Peter’s radical changes.” Peter therefore ordered Alexei to either strive for the succession or become a monk.

Alexei agreed to enter a monastery. But rather than actually do so, he borrowed money and fled the country in disguise, accompanied only by Afrosina (who was dressed as a male page) and three servants. Showing up in Vienna, Austria, in November 1716, he placed himself at the mercy of the Habsburg Emperor Charles VI, who was married to the sister of his deceased wife, Charlotte.

Bushkovitch explains that the flight of the tsarevich put the Austrians in a complicated position. On the one hand, Charles VI had no wish to provoke a fight with Russia. But on the other hand, he felt obligated to respond as Alexei’s brother-in-law and was no fan of Peter. “Austria is still a major power in the 18th century,” Bushkovitch says, “and they don’t like the fact that Peter, by defeating the Swedes and by allying with the Danes and Prussians, has become a factor in the politics of northern Germany.”

Ultimately, Charles VI decided to take Alexei in, hiding him first in a castle in the Alps and then later in a castle overlooking Naples. Unfortunately for Alexei, however, Peter’s agents managed to track him down, and in September 1717 they handed him a letter in which Peter lambasted his “disobedience” but promised before God not to punish him as long as he returned to Russia.

Prior to his escape, a confidant purportedly warned Alexei: “Remember, if your father sends somebody to persuade you to return, do not do it. He will have you publicly beheaded.” But the tsarevich ignored this sage advice. Reluctantly crossing back into Russia in early 1718, he fell to his knees in front of Peter and begged for forgiveness as part of a public spectacle in which he was disinherited.

Peter then demanded that Alexei name his accomplices, which led to the torture of dozens of the tsarevich’s associates. A few were executed, while others were banished or imprisoned. Peter even took action against his ex-wife, Eudoxia, confining her to a second, more remote convent and brutally torturing to death her lover.

At this point, Alexei apparently still hoped for a quiet life with Afrosina in the countryside. But even she ended up testifying against him, after which Alexei was jailed, put on trial, and tortured. Most sources state that he was whipped 25 times on June 19, 1718, and that, when the torture started up again five days later, he confessed to conspiring for the death of his father. (Bushkovitch points out that evidence for this timeline is shaky.) On June 26 (or July 7 by the New Style or Gregorian calendar), the tsarevich died of his wounds.

Upon digging through archives in multiple countries, Bushkovitch determined that Alexei had, to at least some degree, plotted against Peter. “There was clearly some sort of understanding with the Austrians that the tsarevich Alexei might be able to lead some sort of revolt,” Bushkovitch says. The Swedes likewise attempted to recruit the tsarevich. Yet these plans never got off the ground. What’s more, Bushkovitch found no sign that Peter’s opponents in Russia were involved. “There were a lot of people who hoped this might happen,” Bushkovitch says, “but they weren’t organizing anything.”

At any rate, even by the bloody standards of royal family feuds, Peter’s cruelty stands out as unique. “So far as I know,” Daly says, “there were no other European monarchs who oversaw the torture of their own children.”