Force harmony was a light side Force power that allowed several Force-users to join their Force energies together in a pure and powerful manifestation of the light side. Bathing them in light side energies, the ability protected users from dark side powers and could sever opponents from the dark side entirely. Jedi Knight Luke Skywalker, his twin sister, Leia Organa Solo, and the unborn Anakin Solo used this ability to separate the reborn Emperor Palpatine from the dark side, causing the Force storm he had summoned to go out of control, consuming him.
Palpatine is severed from the dark side.
Force harmony was a Force ability designated in the families Control, Sense, and Alter that allowed several willing Force-users to manifest the power of the light side. Once in effect, it bathed the users in the illumination of the light side and could act as a shield against dark side powers. The power could be kept up almost indefinitely, although a slight strain was put on whoever initiated the technique. For the duration that the ability was in use, the users would be united by the Force in all its intensity. The energies of the light side would flow through them in waves and give them complete clarity in the Force.
The initiator could only link as many other Force-users to the chain as their Force energy allowed. The power could not cancel out the dark side, but it could distract its practitioners and make their actions more difficult to perform, and in some cases, cut them off from the dark side completely. Force harmony could also interrupt dark side abilities already in use by engulfing their users in waves of light side energy; those powers inherently difficult to control, such as Force storm, were especially susceptible to this interference.
The proximity between users in the chain, strength in the Force, and blood relation were all factors that could affect the intensity of this ability. In order to begin Force harmony, the initiator had to be adept in Life Sense and telepathy. Relatively powerful Derriphans, creatures of the dark side, could be defeated by a group of Force-users in a Force harmony chain by transferring light side Force energy to the creature.
Sidious is consumed by his own storm.
After Emperor Palpatine resurrected himself in a clone body in 10 ABY, he brought the Jedi Knight Luke Skywalker to the dark side and trained him as his apprentice. Skywalker's twin sister, Leia Organa Solo, traveled to Palpatine's flagship, the Eclipse, and brought her brother back to the light side.
Once Palpatine summoned an incredibly destructive Force storm to annihilate the New Republic fleet at Pinnacle Base, Skywalker and Organa Solo brought their Force energies together and enveloped Palpatine in a wave of light side energy. Augmented by their twin bond and the strength of Organa Solo's third child, who was still in the womb, the twins were easily able to cut Palpatine off from the dark side, causing him to lose control of his storm. The storm consumed Palpatine and the Eclipse, but not before Skywalker and Organa Solo escaped the doomed ship unharmed.
Several Jedi Knights and Masters of the Old Jedi Order, such as Ood Bnar, Arca Jeth, Thon, Vima-Da-Boda, Taj Junak, and Yoda, possessed the ability to initiate Force harmony. Additionally, the ancient Dark Lord of the Sith Freedon Nadd, a master of both Jedi and Sith abilities, could conduct Force harmony. Elite members of the Emperor's Shadow Guard led by Inquisitor Valin Draco were also known to possess the ability.
Leia Organa Solo first learned of the ability from the Tedryn Holocron. The holocron's gatekeeper, Bodo Baas, told her of a prophecy related to him by his Master almost a thousand years ago, which predicted that Skywalker and Organa Solo would defeat Palpatine together. Organa Solo realized the meaning behind the prophecy after confronting Palpatine on the Eclipse, and joined her Force energy with Skywalker's. Skywalker, too, realized that the only way to combat Palpatine's dark side energies was through pure light and joined his sister. Combining their powers with Organa Solo's unborn child, Anakin Solo, the twins were able to vanquish Palpatine.
Force harmony first appeared in the sixth issue of the Star Wars: Dark Empire comic book series, written by Tom Veitch, illustrated by Cam Kennedy, and published by Dark Horse Comics on October 20, 1992. It was also referenced in the issue's endnotes. It was later named and given an expansion in Michael Allen Horne's Dark Empire Sourcebook and made an appearance in the audio adaptation of Dark Empire.
The Official Star Wars Fact File 125 incorrectly describes this ability as a "storm of light-side energy," mistaking it for the Force storm that destroys the Eclipse.