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Tread lightly while reading about the 1966 film Incubus, for evil has befallen many who’ve come in contact with it.

Before he commanded the starship USS Enterprise, William Shatner was the lead actor in Incubus, a low-budget, black-and-white horror film directed by Leslie Stevens (creator of the sci-fi anthology series The Outer Limits). Incubus wasn’t your average art house flick: It was filmed entirely in the constructed language Esperanto, one of only two films in history to do so.

Created in 1887 by Polish ophthalmologist L.L. Zamenhof, Esperanto was meant to ease communication between people who did not share a common language in order to foster peace around the world. Today, it has only a handful of native speakers, but 2 million people across more than 100 countries are believed to be fluent. Popular language-learning app Duolingo offers a free course in it.

Incubus did not employ Esperanto to promote world peace. Rather, the filmmakers thought it sounded creepy and might add an otherworldly element to the film. One reviewer said Incubus was “like a foreign film from a country that never existed.”

The film is set in an imaginary village where travelers come to use a magic well with mysterious healing properties. It’s there where Shatner, playing a wounded soldier, meets and falls in love with a succubus.

Shatner and the film’s other actors were not Esperanto speakers. They learned their lines phonetically in just a few weeks, and filmed them without an Esperanto expert on set. Unsurprisingly, the film was slammed by actual Esperanto speakers when it debuted at the San Francisco Film Festival in 1966. Film critics, unaware that the Esperanto pronunciation was atrocious, tended to enjoy the film.

But then things turned tragic.

Ann Atmar, who played one of the film’s succubi, committed suicide shortly after filming ended.

A few months later, Milos Milos, a Serbian actor who played the titular incubus, murdered Barbara Ann Thomason—the estranged wife of comedian Mickey Rooney—and then killed himself in Rooney’s bed.

Then, in 1968, the daughter of another actress in the film, Eloise Hardt, was kidnapped and murdered. Her killer was never identified, but police believed she may have been murdered by members of the Manson family, who would kill actress Sharon Tate a year later. Tate attended the Incubus premiere with her boyfriend at the time, movie director Roman Polanski.

Others involved with Incubus were beset by yet more unfortunate events: Stevens’s production company went under, the music editor was imprisoned for scalping Super Bowl tickets, and most prints of the film itself were destroyed in a fire. Many believed the film was cursed.

The film struggled to find distributors even though it appeared to be widely admired. The strange language was hard to sell, and some companies didn’t want to be associated with the horrific Milos murder-suicide. So it wound up in France, where it was embraced by the country’s art-house film community but soon lost to history.

In 1993, producer Anthony Taylor wanted to release Incubus on home video but couldn’t find a single print. A few years later, a friend located a damaged copy at Cinémathèque Française, a French film archive. He restored the print himself, and with funding from the American cable network Sci-Fi (now called Syfy), Incubus was finally released on DVD in 2001. You can now buy it on Amazon, if you dare.

Incubus didn’t curse everyone, of course. Shatner would go on to star in Star Trek and become one of Hollywood’s most successful and recognizable actors. Cinematographer Conrad Hall would go on to win three Academy Awards for his work on Butch Cassidy and the Sundance Kid, American Beauty, and Road to Perdition.

In his 2011 book Shatner Rules, Shatner wrote that while filming Star Trek a few months after he finished filming Incubus, he was threatened by a group of Esperantists who then put a curse on the film. After that, Shatner said he started destroying every copy of the film he could find. But you can still find it in its entirety on YouTube.