The Barrier of Essai was a rudimentary Dyson sphere encompassing the Essaian system in "the Triangle", a region within the Federation-Klingon Neutral Zone in the galaxy's Alpha or Beta Quadrants.
History and specifics[]
The Barrier was 600 million kilometers in diameter and two kilometers thick, made up of pulverized debris closely fitted together to form a hemispherical wall. It was built from the material remains of planets, moons and asteroids once in the Essaian system. The structure retained large deposits of dilithium.
The Barrier was manufactured in the 17th century. It was built by Essaians to protect their homeworld Essai from the radiation of an impending supernova which eventually formed the Beta Origi nebula. Even in the 23rd century, the Barrier shielded the planet from dangerous radiation. It had the unintended side effect of keeping the system undetected by members of the Federation, Klingon Empire and Romulan Star Empire, who wouldn't expect to find an inhabited system this close to a supernova.
The Barrier was first discovered in the late 23rd century by Commander Kordon sutai-Donatil and Lieutenant Mida tai-Ocapil, survivors aboard the badly damaged IKS K'vinta. (FASA RPG module: The Barrier of Essai)