Cloned from the hand Luke Skywalker lost in his lightsaber duel with Darth Vader on the planet Bespin many years before, Luuke Skywalker was a twisted copy of Luke trained in the ways of the dark side of the force by Joruus C'Baoth. Mainly due to C'Baoth's failure to lure the actual Luke Skywalker under his influence.
When Luke and his future wife Mara Jade confronted C'Baoth in the Emperor's throne room on Mount Tantiss, located on the planet Wayland. Luuke revealed his existence to his new enemies, brandishing the lightsaber that once belonged to Luke and his father Anakin Skywalker before him. Both clone and template then went on to engage in a lengthy duel.
Prior to his death on the second Death Star, Mara Jade had been ordered to kill the real Luke by the Emperor and had suffered from constant mental torment since. However during the fight between Luke and Luuke she exploited a opening in Luuke's moment of distraction, taking up Leia's lightsaber and stabbing the Clone in the chest thus completing the Emperor's Last Command and freeing her from her mental torment.