Being a king or queen has always been a treacherous job. Between homicidal enemies, duplicitous courtiers and back-stabbing family members, royals had every reason to constantly fear for their lives. And there was one form of assassination that particularly terrified them: silent, invisible poison.

For centuries before the age of Enlightenment, paranoid royals sought protection in superstition, alchemy and quackery. They paid enormous sums—sometimes a proverbial king's ransom—for magical objects they believed would neutralize, expose or repel poison. The most coveted of those? The mythical "unicorn horn," also known as an alicorn.

“Before chemistry was a thing, people believed that many objects and foodstuffs had magical ‘virtues’ or properties,” says Eleanor Herman, author of the Royal Art of Poison, whose research documents the intentional poisoning of royals by their enemies—and the protections they employed. "It was only logical that unicorns, being very rare creatures…must have more virtue than any other.”

Rulers believed such items would protect them because that is what the most learned men of the time told them, notes Herman. "These days, world leaders have their Secret Service agents," she says. "Back then, they had their food tasters."

Even the normally rational Queen Elizabeth I of England was a believer. In addition to buying a magnificent spiral unicorn horn, for the lofty price of 10,000 pounds, she was also known, Herman says, to drink from a unicorn horn cup, believing that if poison touched it, it would explode. And she enjoyed an even more coveted specimen, described by historian Jerry Dennis in A Walk in the Animal Kingdom:

The Queen was so impressed with Frobisher’s gift that she ordered it preserved with the British crown jewels.

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Of course, “unicorn horns” didn’t come from mythical beasts—since, being mythical, they never likely existed. Most came from the tusk of narwhals, an Arctic whale possessing a magnificent spiral tusk that can grow as long as nine feet. These remarkable appendages actually serve as sensory organs, allowing the creature to detect subtle changes of temperature, pressure and other atmospheric elements.

The misnomer may have started with Vikings trader who, around 1000 A.D., began finding narwhal tusks washed up on the beach in places like Greenland and selling them to Europeans. The trade strengthened during the Middle Ages, when the unicorn became a symbol of Christ, and therefore an almost holy animal. 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.

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European rulers became obsessed with owning the magical unicorn horns, which become popular as state gifts. In 1533, Pope Clement VII presented King Francis I of France with a magnificent horn mounted in solid gold. Ivan the Terrible had a staff made from one. Philip II of Spain apparently had 12. The royal Habsburg family placed one of their tusks in a scepter covered in gemstones. And in the late 1600s, Christian V of Denmark sat on a throne of unicorn horns, which went on to be used in coronation ceremonies for centuries.

Elizabeth’s successor King James I was a bit more suspicious, according to Nigel Suckling, author of Unicorns. After purchasing a particularly costly horn, James tried it out by giving poison to a servant, followed by an antidote made of powdered unicorn horn. When the servant died, James believed he had been hoodwinked.

Horns weren't the only antidotes royals employed against the dreaded poison. Some used stones etched with scorpions. Others placed gems such as emeralds and amethysts in their goblets. Still others sought protection from powders crushed from bezoar stones (hairballs and other undigestible solid masses pulled from animal stomachs) or toadstones (mythological gems embedded in toad’s foreheads that were actually fossilized teeth of extinct fish).

To stave off poisoning attempts, some royals took a daily antidote, or theriac, to build immunity. Theriac ingredients included common foodstuffs like parsley, carrots, black pepper, cloves, wine and honey, says Herman. Others ingested sulfur and garlic, now known to neutralize arsenic in the bloodstream. And, she added, “Some theriacs included real poison such as arsenic in minute amounts to get the body used to it slowly, so that a single large dose might not prove fatal.”

What’s ironic in all this is that royals—along with the general population—poisoned themselves daily in countless ways. Elizabeth I probably hastened her death by her constant use of lead-based white face paint; in her last year, she showed many signs of lead poisoning. Cosmetics and medications contained large amounts of mercury, lead, arsenic, animal and human feces and urine, and dead body parts, says Herman.

And that’s not counting all the banal, insidious ways people poisoned themselves. “I imagine many royals were poisoned with infection of many kinds,” Herman says, pointing to the lack of regular bathing or sanitary plumbing facilities. “Henry, Prince of Wales, died at the age of 18 in 1612 from typhoid which he got either swimming in the river or eating oysters.”

Indeed, many people likely died of food-borne illness, due to lack of refrigeration and thermometers, and hard-to-control hearth cooking. “Food poisoning, which must have been rather frequent, has almost all the same symptoms as arsenic poisoning,” says Herman.

The reverent belief in the curative and preventative properties of unicorn horns and gemstones began to dissipate as the Enlightenment brought advances in scientific experimentation. By the late 17th century, magic, alchemy and astrology were slowly replaced by chemistry and science. As unicorn horns and other poison remedies underwent repeated testing, old superstitions began to fall away.

Interestingly, a few of those superstitious poison antidotes did turn out to have scientific support. One, says Herman, was clay from the Greek islands of Lemnos and Samos called terra sigillata: “The substance contains silicate particles which attract the metals of metal-based poisons such as arsenic. The clay then carries them out of the body, preventing them from fully absorbing into the bloodstream." Also effective: those fossilized sharks' teeth called toadstones. The calcium carbonate in the fossils, she says, can neutralize poison.

Today, unicorn horns can still be seen in royal collections across Europe—but now simply as an impressive decorative curiosity.