Luke returns to Dagobah, and rediscovers lost ancient ruins of Jedi temples. It is atop this site that he chooses to build the new Jedi temple, far from the political center of Coruscant, and close to its historical roots. It was these ruins he entered, unknowingly, when Yoda sent him to experience his first taste of the dark side in the cave.
Under the new temple, In this ancient place, Luke experiments with the dark side in an attempt to re-unify the two sides of the force. He does this under the guidance of Yoda and Anakin, as well as other long-dead Jedi.