As "Jihadi John," he was a terrifying figure, his identity concealed by a black mask, his threatening tone backed up by his oversize, serrated knife and his willingness to use it in the name of Islamic State and its self-declared caliphate.
His professional-looking videos began with a political rant and ended with his victims lying dead at his feet, severed heads cupped in the sands of Syria. He seemed both judge and executioner, savoring each fresh kill.
After the 9/11 attacks on the US, many believed that terrorists would turn to crude weapons of mass destructions to attack cities. Few predicted that a man with a knife and a video production team could have such an impact using a medieval technique.
Now that he has been exposed as Mohammed Emwazi, the tall man with the British accent and mocking tone is no longer a mystery. He is revealed as one more furious young Londoner, in this case a well-educated, middle-class jihadi in his mid-20s who turned against his adopted country after he moved to Britain from Kuwait as a boy.
His unmasking may well have reduced his usefulness to the cause.
For one thing, with his identity known, and the global distribution of pictures of him looking slightly goofy in an ill-fitting Pittsburgh Pirates baseball cap, Emwazi may become less sinister to viewers, less able to send chills up the spines of people who abhor Islamic State's claim to be killing civilians in the name of Islam.
If he kills again on camera, the element of surprise will be gone and the reaction may well be, "Oh, him again."
Also, now that authorities know who he is, there is little doubt he will become the target of a drone attack if the US or Britain can learn his precise whereabouts. The pressure on him could make him less valuable to Islamic State militants - perhaps even a liability.
Magnus Ranstorp, a terrorism specialist with the Swedish National Defense College, said Emwazi can be expected to play a reduced role in the organization because every time he speaks on a mobile phone he risks having his location pinpointed, sparking drone fire that could kill him and others. Ranstorp said the identification of Emwazi also gives the public hope that he will be brought to justice.
"It's quite important for families of the victims," he said. "They know where to focus. They know there is one particular person who has been named who authorities will focus on and who will live for the rest of his life knowing that every day he will face a possible drone attack. Now that he is known, he may not be as menacing as he once was."
Now that details about his personal trajectory have begun to emerge, Emwazi becomes the stuff of parliamentary inquiries: How was he radicalized? Why didn't the security services determine he was a mortal threat and do something to keep him from getting to Syria?
Emwazi is perhaps the most chilling exemplar of the radicalization trend that is gaining pace not just in Britain but also in France, Belgium, Denmark and other countries in western Europe.
He went to Syria early, in 2013, in the vanguard of the British jihadi movement, before the Islamic State militants seized territory and issued a call for other likeminded people - including girls and young women - to join its ranks in Syria and Iraq.
There is circumstantial evidence suggesting Emwazi tried earlier to link up with al-Shabaab terrorists in Somalia but was thwarted in part by a British spy who tried unsuccessfully to recruit him into the secret service.
Since then, the call to jihad has intensified, galvanized in part by easy access to Internet sites that depict the Islamic State's territory as a religious utopia governed by Shariah law.
Britons watched helplessly this week as three teenage schoolgirls who had run away from their homes in London were reported by police to be in Syria, apparently linked up with Islamic State extremists as potential "jihadi brides."
Now al-Shabaab is making threats of its own, warning that the two big shopping malls in London - as well as the famous department stores on Oxford Street - are considered targets for terrorist attacks along with the Mall of America in the US.
Nearly half of British Muslims surveyed in a BBC poll published this past week say the British public is becoming less tolerant of Muslims. At the same time, the UKIP political party is making gains by taking a stand against increased immigration.
This increased polarization was clearly one of the goals of the Islamic Stage campaign that used Emwazi's familiar London accent as a potent reminder to Britons that the enemy was in their midst: not some far-off person speaking Arabic, but a homeboy from their streets.
"The fact that he appears like a relatively ordinary young British resident is disquieting," said John Gearson, professor of national security studies at King's College London. But "the de-mystification of this individual reduces the propaganda effect for Islamic State. He's just a murderer now."
Still, if Emwazi's moment has passed, Islamic State militants - with their strong grasp of how to use social networking and video to spread fear - are likely to come up with other ways to shock the public.