According to wikipedia, there are only four full-length films shot entirely in Esperanto. One of these four is the 1966 black-and-white horror film Incubus, starring William Shatner.
The film had an LA premiere, but then, partly because of the Esperanto dialogue, it never found a distributor except in France and fell into obscurity. For years it was believed that all copies of the film had disappeared, until the 1990s when a copy was found in France.
The whole thing takes on the air of a ninety minute game of "Clue" played in reverse, as greedy, grasping relatives and hangers-on are dispatched in the most gory and inventive ways possible for a dead guy to come up with. Amongst the suspects/victims are THE Elsa Lanchester, Roddy McDowall, Patric Knowles, Farley Granger, the witchy Ms. Wallis, who gets to sample some of the hottest "cold cream" on the market, Jamie Farr (yeah, Klinger from M*A*S*H himself,) and Victor Buono, in a hysterical cameo as the 'lucky' minister who gets to officiate at the nuptials.