The Force ghost was the soul and essence of a deceased Force-sensitive who denied the will of the Force upon death, yet was able to interact with the living. It also described those who committed a final act of selflessness that led to their death, and had come to terms with it. Some Sith Lords learned similar techniques, which in some cases allowed them to physically interact with their environment.
While most Jedi would lose their bodily envelope and become one with the totality of the Force upon their death, some had learned a technique allowing them to retain their own identities separate from the Force for a time. Those individuals could linger in the physical world in the form of “Force Spirits,” nonphysical entities that exhibited most traits and abilities they had in life. This ability did not have to be learned, and could be achieved through denying the will of the Force upon death, or committing a selfless act of sacrifice. According to the knowledge contained in Asli Krimsan‘s holocron, that feat could mainly be achieved by sublimating one’s organic cells into a state of pure energy.
In its “normal” state, a Force Spirit could only interact with other non corporeal entities. In order to be heard or seen by the living, the Spirit had to manifest itself. There were different types of manifestation. On a basic level, a Force ghost could appear as a floating, milky gray mass of energy that lacked distinctive features. But it could also manifest itself as the exact image of the dead being during their lifetime, only transparent and emitting a bluish glow. While it could appear to anyone it wished, a spirit could not choose to become audible or visible to only some beings and not the others, since the ability to perceive the manifestation depended on the beholder’s strength in the Force. While many Force ghosts could appear as a vision of their former selves unmarked by injury or disease, others still bore the marks of what killed them. The ghost of Aidan Bok wore tattered robes and had gaping wounds in its body, the marks left by a lightsaber. Sometimes, a particular Force ghost would bind itself to a Force-sensitive as a guardian spirit for the purpose of providing wisdom, continuing training, and offering aid.
In theory, Force spirits could neither harm or be harmed by physical entities, due to their spiritual nature. For the same reason, they could pass through solid matter effortlessly and were never hampered in their movements. While some spirits would simulate the walk of the living beings, subject to gravity, others would hover and drift above ground level. However, in some cases, a spirit could acquire temporary solidity by drawing on a living creature’s link with the Force, which allowed it to actually touch that creature. Notably, the ghost of Aidan Bok could make itself solid enough to shove Tash Arranda through a doorway.
A manifested spirit could wield Force powers, although not all of them. It also had the ability to travel instantaneously to any point in the galaxy by force of will. That power, however, required that the spirit be familiar with the destination. The presence of the Force at the place of arrival was also necessary for that ability to work, since it acted as a beacon.