Duel on Dagobah (3 ABY)

A duel took place in 3 ABY in the Dark Side Cave on the planet Dagobah, between Luke Skywalker and the Dark Spirit, who had assumed the shape of Darth Vader. It was a part of Skywalker's Jedi training and occurred because his Jedi Master Yoda had asked him to enter the cave so he could face his darkest fears. Beforehand, when Skywalker inquired whether there was anything that posed as a threat, Yoda cryptically replied that the only thing to threaten him was "what Skywalker carried in with him," and also advised that Skywalker not bring his weaponry inside as he won't need them. Skywalker seemingly got the upper hand, beheading the apparition. However, when the fake Vader's helmeted head hit the ground, its mask exploded, revealing the face of Skywalker himself.

Text and facts from the “Duel on Dagobah (3 ABY)/Legends” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.