Force spirit

"You will learn to preserve your Life Force, and so, manifest a consciousness which will allow you to commune with the living after death."

- The spirit of Qui-Gon Jinn, communing with Yoda

Force spirits were beings who gained immortality through the preservation of their consciousness after death. When lifeforms died, they became one with the Force. Through special training, however, deceased Force-sensitives could retain their identity and, therefore, communicate with the living. By achieving spiritual immortality, a person's physical form would vanish upon their death. This knowledge could be learned only by those who fully embraced the light side of the Force, such as the Jedi who lived by a code of selflessness and sacrifice. The followers of the dark side of the Force, such as the Sith, were incapable of learning this power.

Before the Fall of the Republic, Jedi Master Qui-Gon Jinn learned the ability to become a Force spirit, but was killed by Darth Maul before he could complete his training. As a result, he could commune with the living only as a disembodied voice. Near the end of the Clone Wars, a conflict in which the Galactic Republic and Jedi Order were at war with the Separatists, Grand Master Yoda embarked on a spiritual journey that brought him to the planets Dagobah and Moraband&mdash;as well as the birthplace of the midi-chlorians&mdash;and ultimately learned how to retain his identity after death. This ability was also learned by Jedi Master Obi-Wan Kenobi during the rise of the Galactic Empire. Although Anakin Skywalker&mdash;a fallen Jedi Knight and the Chosen One of the Force&mdash;never received the training that allowed his mentors to become Force spirits, his redemption and sacrifice made it possible for his consciousness to be preserved after death by Kenobi and Yoda.

Description
Through the Force, certain beings with special training were able to appear as Force spirits. The knowledge was open only to those who followed the light side, so the only known individuals with the ability to manifest after death were Jedi. The dangerous level to become a Force spirit was low, but only possible for a few rare Jedi. It was made possible by energy from the Living Force flowing into the Cosmic Force. The Force Priestesses, who dwelled on the planet, said to be the origin of the midi-chlorians, taught the skill to Qui-Gon Jinn, the first known Jedi to learn the secret. Unfortunately, his training was incomplete upon his death, so he could only manifest as a disembodied voice. However, Qui-Gon was able to complete his training even in death, appearing to Obi-Wan Kenobi on Tatooine in an almost corporeal form. The secret was later passed on to Obi-Wan Kenobi, Yoda, and Anakin Skywalker.

Force spirits were not only able to appear anywhere in the galaxy with a strong presence in the Force, but were also able to use it at a much stronger degree than most living beings. This was demonstrated by Yoda's capability of either manipulating weather to create lightning strikes, or summoning such strikes himself. Additionally, he was also shown to be capable of striking Luke Skywalker with his walking stick, demonstrating an ability to physically interact with the living world despite both Yoda and the stick being ostensibly non-corporeal.

The Sith were unable to manifest themselves after death, causing them to concentrate on gaining earthly power only. One Sith, the Presence, was able to preserve part of her conscious essence so long as it was bound to an object like a Sith holocron on the Sith temple of Malachor. Lord Momin had part of his consciousness preserved in his mask.

Behind the scenes
"In the rough draft...Ben explains that...if Vader becomes one with the dark side of the Force, he will lose all identity. If he turns to the good side, he will pass through the Netherworld and" in the revised rough draft, Yoda "will rescue him before he becomes one with the Force"."

- Description about preserving identity after death in Episode VI's rough draft

In the rough draft of the 1983 film Star Wars: Episode VI Return of the Jedi, it was revealed by a resurrected Obi-Wan Kenobi that if Darth Vader became one with the dark side of the Force, he would not be able to retain his identity after death.

Non-canon appearances

 * Star Wars
 * LEGO Star Wars: The Force Awakens
 * LEGO Star Wars: The Force Awakens
 * LEGO Star Wars: The Force Awakens
 * LEGO Star Wars: The Force Awakens