Well of the Dark Side/Legends

The Well of the Dark Side was a volcanic crater on the planet Mortis that bottomed into a lava lake. The Son resided here where he drew his strength in the Dark Side of the Force. Upon learning that the Son was planning to escape Mortis, Anakin Skywalker followed the instructions of a vision of Qui-Gon Jinn to go to the Well and learn his fate. Skywalker heeded his old master's advice and rode his speeder bike to the bottom of the Well. There, he encountered the Son, and was shaken by a vision of his future. From this point, Skywalker followed the Son in order to avoid these heinous acts, and they both left the sanctum of the Well.

Behind the scenes
The Well of the Dark Side was first seen in "Ghosts of Mortis," an episode in the third season of the Star Wars: The Clone Wars television series, and the third in an arc of three episodes. The fiery location drew heavily inspired from scenes on Mustafar in Star Wars Episode III: Revenge of the Sith. Concept design for the Well of the Dark Side originated from early Ralph McQuarrie sketches from Star Wars Episode VI: Return of the Jedi, of subterranean levels of Imperial Palace on Coruscant where an encounter between Luke Skywalker and Palpatine would take place.