The ancient prototype lightsaber known as the proto-saber was referred to as the laser sword by the Mustafarians. This weapon was secured within an Imperial base on Mustafar, situated in proximity to Fortress Vader. In the year 1 BBY, operatives from the Rebel Alliance embarked on a mission to Mustafar where they came across the proto-saber.
Unlike the lightsabers utilized by the Jedi Order during the Clone Wars, the proto-saber had a different method of operation. Rather than projecting a plasma blade, this weapon had two distinct physical prongs that extended from a crossguard. Upon activation, energy would flow through these prongs, creating a blade that formed between them.
In 1 BBY, the Rebel Alliance initiated a mission to the planet of Mustafar. Captain Cassian Andor, having received intelligence about a hazardous artifact located there, dispatched a team of agents to a secret Imperial base. Inside, they found the proto-saber, their intended target. Their pilot, Athex, began analyzing the device to transmit its data to the Alliance. Unexpectedly, the sword flew from the agent's hands and into the grip of Darth Vader, who emerged from the darkness and launched an attack on the rebels. As the other agents provided covering fire for Athex, Vader used the Force to pull the pilot toward him and stabbed him with his lightsaber. Just as Vader was about to eliminate the remaining rebel operatives, K-2SO, a reprogrammed security droid, crashed a shuttle into the structure, giving the agents time to escape. After reaching space, the agents transmitted the proto-saber's data to the Rebel Alliance.
The proto-saber made its debut in Star Wars: Secrets of the Empire, an immersive hyper-reality experience produced by The VOID and ILMxLAB. It premiered at Walt Disney World in Orlando, Florida on December 16th, 2017, followed by Disneyland in Anaheim, California on January 5th, 2018. A limited engagement also took place in London, running from December 16th, 2017, until March 10th, 2018.
David S. Goyer, the writer of Star Wars: Secrets of the Empire, officially identified it on January 31st, 2018, during an appearance on The Star Wars Show. Goyer mentioned in an interview, "That object, which we are currently referring to as the proto-saber, is intended to be a prototype lightsaber, and it may eventually appear in other aspects of the Star Wars universe."
Despite the absence of any connection within Secrets of the Empire, the 2020 reference book, Star Wars: The Lightsaber Collection, mistakenly associated the proto-saber with the lightsaber wielded by the Black Bishop, a character featured in Vader Immortal, another VR project by Goyer. Matt Martin of the Lucasfilm Story Group later clarified via Twitter that these were distinct weapons, suggesting that the proto-saber likely belonged to Lady Corvax, the Bishop's wife.