During the time of the Old Republic, a group of Jedi Masters defeated a Dark Jedi, who wielded a dangerous artifact known as the Soulsaber. They constructed the Cavern of Light to hide it in, using the power of the light side of the Force to prevent it from being taken.
During the Galactic Civil War, the Dark Jedi Thaum Rystra traveled to the cavern in order to steal the Soulsaber. He brought with him Caeleb, a Force-sensitive child whom he used to help retrieve the Soulsaber, as Rystra could not get it on his own, due to its protection by the light side. However, a group of Rebel Alliance agents attacked Rystra in the cavern. He could not defeat them, so he abandoned Caeleb and fled with the Soulsaber.