Star Wars has finally explored the hidden Sith shrine that lies beneath the Jedi Temple on Coruscant. The first Jedi Temple was established on the planet Ahch-To, on the fringes of the galaxy. Over the millennia, though, the Jedi became increasingly bound to the Republic, and ultimately the Temple on Coruscant - the galactic capital, believed to be the planet on which humanity evolved in the Star Wars galaxy - became the most important of them all.
The Coruscant Temple was a massive building, towering over the city planet itself, visible for miles around. It stood as a potent symbol to the power and grace of the Jedi, a beacon of light and hope to citizens and visitors to Coruscant alike. Along with the Senate itself, it was one of the two centers of power in the galaxy, for the Jedi traditionally stood a little apart from the government. Unfortunately the Jedi and the Republic ultimately became practically synonymous, leading to a bitter power struggle between the Jedi and Chancellor Palpatine, a Dark Lord of the Sith.
But the Jedi Temple was secretly built atop a powerful vergence in the Force, one that had originally been claimed by the Sith and had consequently been transformed into a Sith shrine. When the Sith were driven back from Coruscant, the Jedi claimed the vergence for themselves, and established first a shrine of their own and then the Temple. Incredibly, Claudia Gray's novel Into The Dark - a story set at the peak of the High Republic Era, 200 years before the events of Star Wars: Episode I - The Phantom Menace - reveals the Jedi actually used the vergence. They would take relics and other objects strongly imbued with the dark side down to the vergence in order to perform rituals of purification, stripping the dark side from these objects and depositing it within the vergence. They believed it would then dissipate into the cosmic Force.
Curiously, though, by the prequel era, the Jedi appeared to have stopped using the ancient Force vergence for this purpose. Cavan Scott's Star Wars audiobook Dooku: Jedi Lost revealed the Jedi of that time had taken to keeping dark side artifacts in a hidden vault in the Temple, as part of what they called the Bogan Collection. Dooku and Sifo-Dyas broke into the Bogan Collection when they were just initiates, with both given an exposure to the dark side that set them upon dark paths. It included a collection of Sith scrolls and swords, artifacts relating to the Sorcerers of Tund and the Yacombe, and even the helmet of the Sith Lord Darth Momin. Palpatine claimed these Sith artifacts from the Bogan Collection after the fall of the Jedi Order in Star Wars: Episode III - Revenge of the Sith, and it is possible Darth Momin's mask - imbued with the essence of the Sith Lord himself - helped him learn how to survive death.
The interesting question, though, is just why the Jedi stopped purifying objects in the vergence beneath the Temple. It is possible they learned they had been mistaken in their assumption the energy they drew out of the various relics dissipated into the cosmic Force, and that instead they were strengthening the power of the dark side beneath their own Temple. That would explain why they abandoned the vergence altogether, sealing it away and stopping teaching members of the Order about it, so that it had been forgotten by all but the older Masters by the time of the prequel trilogy.
