For centuries Europeans sought out the “unicorn horn” — the long and straight tusk of the arctic-dwelling narwhal whale — for its perceived magical and curative capabilities.

On Friday, the tusk did indeed wield a historic force, just not in the way the queens and kings who once collected the twisted and tapered ivory likely imagined.

Instead, as the deadly knife attack unfolded on the London Bridge, a man, described in news reports as a Polish chef, grabbed the nearest arms he could find for self-defense — a narwhal tusk — and headed to help stop the melee.

The simple, heroic act in a way embodied the ancient lore of the larger-than-life tusk.

The attack began by Fishmongers’ Hall, a historic building and events space on the London Bridge replete with ornate decorations, two massive narwhal tusks included. The assailant, 28-year-old Usman Khan, had been at the hall attending a conference when he wielded a knife and fatally stabbed two people before police shot him dead.

Amid the shock and horror, it struck many as quintessentially British the way the beloved “unicorn horn” suddenly surfaced to help ward off the attacker, said British historian and journalist Guy Walters.

“There’s something very British about fighting a terrorist with something as surreal as a narwhal tusk,” he said. “We don’t carry weapons in this country. But we do have narwhal tusks around."

For centuries Europeans thought that the elusive narwhal whale was a “sea unicorn” with mystical abilities

Walters described the draw of the strange whale and its sharp tooth as “enigmatic and weird” and even “almost humorous."

“The beauty of narwhals is that they were always these really mysterious creatures,” he said. “For centuries no one really knew what they were. When they found these tusks they assumed that they were unicorns. … They became these incredibly sought-after items."

The tusk is in fact just a tooth but a rarity among whales for how long (up to 10 feet) and spirally straight it can grow. It’s also incredibly strong and can cut through inches of wood, Walters said. The tusks primarily appear on males, who can sometimes even have two. Scientists have found that the tusk-of-a-tooth has up to 10 million nerve endings and sensory capabilities, as well as possible courtship uses.

Still, there’s much that remains mysterious about the blubbery whale and the majestic tooth projecting from it.

As Katherine Rundell reported in the London Review of Books, “Named rather ungallantly for the Old Norse word nar, meaning ‘corpse’, and hvalr, ‘whale’, after their mottled grey markings, narwhals are unicorn-like not just in their appendages, but in their elusiveness; they are one of the mammals about which we know least. They spend the winter months dodging dense pack ice, where humans cannot follow, and can swim a mile deep, twisting upside-down as they descend into pitch-black water.”

Today, the whales are also under threat, as climate change causes the ice covers that they rely on for shelter and feeding to shrink.

The Vikings, Ivan the Terrible and Elizabeth I, among others, all sought out the powers of the narwhal tusk

For centuries, Europeans sought comfort in superstitions and alchemy — the alluring unicorn and its aquatic counterpart, the narwhal whale, included.

The tusks had long been part of Inuit culture. Then starting around A.D. 1000, Viking traders began selling the tusks, which they found surfacing on the shores of places like Greenland, to other Europeans. Historians have found evidence of the tough tooth being fashioned into weapons for hunting and fighting.

“The trade strengthened during the Middle Ages, when the unicorn became a symbol of Christ, and therefore an almost holy animal,” reported Hadley Meares of the History Channel. “By the Renaissance, unicorn horns had developed a reputation as a poison cure-all, and their cost inflated to ten times their weight in gold — or more.”

Ivan the Terrible, the 16th-century Russian czar, had a ruby, diamond, sapphire and emerald encrusted narwhal tusk that he sought on his death bed, according to Rundell.

Elizabeth I, also of the 16th century, reportedly had several, including one that would be valued at the equivalent of 2 million pounds today, Walters said. Philip II of Spain tried to out-magic everyone by reportedly owning 12.

To test the authenticity — and confirm the supernatural properties — of a “unicorn horn,” people used to break off or scratch the tip and then place spiders by it. In records of the test, the arachnids then died.

The magic of the narwhal lives on

In 2013 a man in Cornwall, England, named John Jeffries sold a narwhal tusk for 36,000 pounds [$46,000 in today’s rate] at an auction, according to Cornwall Live.

But that wasn’t all. “The massive piece of ivory … was presented to a sailor called Cornelius Fudge in 1881. … More than a century later J.K. Rowling gave the same name to the Minster for Magic in her Hogwarts novels,” Cornwall Live reported.

Jeffries had bought the 2.5-meter-long tusk for just a couple of thousands of pounds 40 years earlier, he said. But the enduring allure of the tusk — and perhaps the tie to Harry Potter — dramatically raised its value.

Rowling sent Jeffries a letter, saying that she found the shared name “utterly extraordinary!” and “had never come across” the name’s previous existence, according to the local news report.

Indeed, the coincidence was almost like magic.