Force lightning, also known as Sith lightning or dark rays, was a power that Force users could conjure through the dark side of the Force.
Force lightning first appeared the canon continuity in the 1983 film Star Wars: Episode VI Return of the Jedi, the final installment of the Star Wars original trilogy. In Legends, conversely, Splinter of the Mind’s Eye is the first work this power is introduced in.
As sound designer Ben Burtt wanted sounds for sparks and artificial lightning akin to those in Frankenstein films during the production of the 1977 film Star Wars: Episode IV A New Hope, Burtt tracked down Ken Strickfaden, the person who built and kept the props of Frankenstein, and asked if he could record sounds from the still-functional Frankenstein props. Strickfaden initially denied the request, but, during the production of the 1980 film Star Wars: Episode V The Empire Strikes Back, was stunned after watching the 1977 Star Wars film and drove Burtt to his house in Santa Monica, switching on all his props and allowing Burtt to record the sounds. Much of the Emperor’s Force lightning attack on Luke Skywalker in Return of the Jedi made use of sounds recorded from Strickfaden’s Frankenstein props, with the sound effects being used in subsequent Star Wars media.
George Lucas originally conceived of Force lightning as being just another Force power and wanted Luke Skywalker and Darth Vader to use it against each other in Empire Strikes Back, with Luke seeming to turn close to the dark side to increase his power at Vader’s urging, but ultimately being defeated as in the final film, though ultimately this idea was not implemented in The Empire Strikes Back.