Sometime after the events of the Hundred-Year Darkness and the banishment of a rogue Jedi to Moraband, a war erupted between the Sith Order and the Galactic Republic, leading to Coruscant becoming a contested planet fought over by both sides.
Around 5000 BBY, a Jedi who had gone rogue turned against the Jedi Order. This individual was convinced that embracing the dark side of the Force was essential to achieving their ultimate potential. This Jedi was exiled during an occurrence known as the Hundred-Year Darkness. As a consequence, the Sith were created.
During an era when the Sith held sway over the galaxy, they seized control of Coruscant, the capital world of the Republic. The Sith erected a massive black shrine over a light side vergence situated on a mountain at the center of the planet at least by approximately five millennia prior to 14 BBY. Eventually, the Jedi besieged the shrine, razing it and defeating the Sith forces.
The Jedi then constructed their own shrine upon the ruins of the Sith shrine. In time, the Four Masters built the Jedi Temple above the previous location of the shrine in order to contain the dark energy that was radiating from the corrupted vergence, and symbolically inter the Sith legacy. Over the subsequent centuries, the Jedi Temple gradually took shape. The destroyed foundations of the Sith shrine were progressively covered by layers of plasteel and ferrocrete as a large, flat ziggurat was constructed around them.
Despite the Jedi's triumph in the conflict, their battles with the Sith would continue for millennia thereafter. Although the Jedi mistakenly believed the shrine had been obliterated, a portion of it persisted underground, its malevolent influence contributing to the near annihilation of the Jedi Order roughly 5,000 years later.
Within Star Wars Legends, the initial confrontation between the Jedi and Sith after their original schism was known as the Great Hyperspace War. This war first appeared in Legends in Tales of the Jedi – Dark Lords of the Sith 1, which was published on October 1, 1994.
The Nexus of Power sourcebook makes direct reference to a canon Great Hyperspace War. However, since it does so within the framework of the in-universe mythical Qel-Droma Epics, and there's no mention of an assault on Coruscant, it remains unclear whether this is the same conflict in canon.