Bosthirda was a planet of the Sith Empire, close to Korriban and Dromund Kaas, and was located on the Kamat Krote.
History
Once a part of the Sith Empire, in the following millennia its location was unknown to both the Galactic Empire and the New Republic. Apparently not even Palpatine knew its location. The planet was the location of a dark side temple, the hidden retreat of Supreme Prophet Kadann. Prior to the Battle of Endor, the Supreme Prophet had a falling out with the Emperor. Kadann and his most faithful followers retreated into hiding on Bosthirda, maintaining only the most circuitous of contacts with the greater galaxy. They had little need of news, foreseeing most important events for themselves through Force visions.
Eventually, the Prophets on Bosthirda were massacred by Azrakel, Lumiya, and Carnor Jax.
Behind the scenes
Bosthirda was a planet fought over in the Dark Crusade sequel competition, "Fading Light". All units of the Dark Brotherhood ventured to the planet to retrieve the artifact known as Revelation, the sword of Ferran. Based on competition results, Taldryan was given the right to own the sword.