Can a Sith become a Force ghost? No. As far as I know, only a Jedi that is properly attuned with the Force can offer themselves to it and become part/one with it. Qui-gon, Obi-wan, Yoda, Luke, Leia, Anakin, etc.
Also, the Sith Lord Ajunta Pall lived on through the Force in such a manner as a Force ghost, though it is relatively unknown how or why he remained. However, it can be speculated that his extreme rage and anger made his presence linger.
However, Palpatine was never able to achieve the immortality that the Jedi possess. His plans brought him closer than any other Sith to becoming a Force Ghost. But his failure proves that so long as the darkness outweighs the light, the Dark Side will never lead to immortality.
Qui-Gon Jinn learned to become a Force Ghost by training with Force Priestesses and mastering the ability to appear after death. Jedi must be fully committed to the light side to perceive Force Ghosts, while the Sith cannot become true Force Ghosts but can cheat death.
Naturally, only those with a strong affinity to the Force can achieve the posthumous transformation, and the technique was pioneered by Qui-Gon Jinn, though he was killed before perfecting the technique. Fortunately, Yoda picked up Qui-Gon's studies and, along with Kenobi, mastered the Force ghost form proper.
In material that is now considered Legends, Qui-Gon is even described as a Gray Jedi. Additionally, the term has also been applied to Force-sensitives who aren't affiliated with the Light or Dark side of the Force. Quite literally, these Gray Jedi don't see things in black and white, or light versus dark.
Qui-Gon Jinn appears to Kenobi for the first time as a Force spirit. The first known Jedi to learn the secret was Qui-Gon Jinn, who learned it from a Shaman of the Whills. His training remained incomplete at the time of his death, so he could only manifest as a disembodied voice.
These characters include Ahsoka Tano, Kanan Jarrus, Luminara Unduli, Aayla Secura, Mace Windu, and Adi Gallia. While those mentioned have never appeared physically as a Force ghost, their Force connection with Rey can surely be classed as a Force ghost appearance in Star Wars canon.
The closest that Star Wars canon has come to naming who trained Yoda was revealing that Jedi Master Qui-Gon Ginn was the one who taught Yoda to become a Force Ghost and attain immortality.
Anakin's spirit did return to help Ahsoka overcome her past trauma and to make amends with his former Padawan for the terrible things he did as Darth Vader. The last shot of Ahsoka season 1 presents the scene from his perspective, removing all doubt that he was only in Ahsoka's head.
Somehow, Obi-Wan or Yoda taught him how to become a Force ghost in time for Luke to see him in Return of the Jedi. Another theory stated that Kylo couldn't become his own Force ghost because he and Rey were part of a Force Dyad, meaning that his consciousness was absorbed into Rey when he died.
Darth Vader can not use force lighting, because of his severed arms. Robotic hands can't summon the force lighting. However Darth Vader can still use other forms of the force, such as force choke.
Praven was a Sith pureblood male who served the reconstituted Sith Empire as a Sith Lord and later protected the Galactic Republic as a member of the Jedi Order.
It is possible for a player Sith Warrior or Sith Inquisitor to be a Light Sith. In the Inquisitor's case, their approach leads them to earn the title of Darth Imperius, due to their light-sided actions being viewed as pragmatic acts that serve the Empire in the long run.
Blue in color, Sith shoot Force lightning from their hands by calling on their hatred and aggressive feelings. However, while a deadly weapon, it is not unstoppable.
However, Grogu was captured by Imperial forces, and Djarin was forced to lead a mission to rescue him. After being reunited with Grogu, Djarin delivered him to Luke Skywalker, a Jedi who agreed to take the child into his care and train him in the ways of the Force.
Summary. The ending of The Rise of Skywalker sees Leia as a Force ghost, but it's never explained how she was trained to do this after her death. Leia's ability to become a Force ghost isn't far-fetched, as she had three Jedi Masters who could have taught her - Luke, Obi-Wan, and Yoda.
CANON: Only the Jedi can become Force ghosts, and at least 12 Jedi have done so. They can speak to someone who is strong in the Force, but can only be seen by someone who knew them. They may be immortal.
The term “gray Jedi” in Star Wars is somewhat of a misnomer, considering that the definition refers to one who is practically non-Jedi. That being said, quite a few Jedi can be placed into this category. But Mace Windu identified as a Jedi despite his possible inclusion in the "gray" category.
Summary. Qui-Gon Jinn's refusal to conform to the Jedi Order's wishes posed a threat to Palpatine's plan to take over the galaxy. Qui-Gon's strong connection to the Force would have allowed him to detect Palpatine's evil plans much earlier than the rest of the Jedi.
Dooku wasn't. In the current canon, you have to have fully embraced the Light Side in order to become one with the Force, and you have to learn how to become a ghost (which you can learn after death. So as it currently stands, in the current canon, it's not possible for Dooku to ever become a ghost.
Legend had it that Yoda—a Jedi who became Grand Master—was trained by N'Kata Del Gormo. A Force-sensitive Hysalrian, N'Kata Del Gormo was trained in the ways of the Force and achieved the rank of Master within the Jedi Order.
