Eye of Harmony: 

The Eye of Harmony is source of power that sustained Gallifrey and was, according to the ancient records, the nucleus of a black hole. History says that Omega, a solar engineer who was trying to provide an energy source for the Time Lords' time travel experiments, harnessed the power of a supernova. Omega was sent hurtling into the newly created black hole and he was presumed lost. Some accounts suggest that Rassilon, another member of the project, misled Omega into believing he would survive this mission. Some say it was sabotage. Unknown to anybody for millennia, Omega was not dead, but merely trapped beyond the event horizon of the black hole in an antimatter universe. Rassilon and the Other, the third of a founding Triumvirate of Time Lord society, then took control of the project, and the power of the black hole. Rassilon captured and placed the black hole beneath the citadel of the Time Lords on Gallifrey, known as the Panopticon. He placed it in a perfectly "balanced" condition, so that as long as the Eye existed, the power of the Time Lords would "neither flux nor wither". However, it remained there for so long that it passed into legend and the Time Lords had forgotten its location. Some believed it to be mythical or no longer in existence until the Fourth Doctor and the renegade Time Lord known as the Master rediscovered it. The Eye was linked to two other Gallifreyan artifacts, the Sash of Rassilon and the Great Key of Rassilon. The Sash was needed to control the Eye safely without being sucked into it, and the Great Key was an ebonite rod that could be used to drain energy from the Eye. The Great Key was later renamed the Rod of Rassilon to distinguish it from the other Great Key, a component of the De-mat gun. The Master tried to use the Eye and the relics to give himself a new set of regenerations at whatever cost, even the destruction of Gallifrey. He was stopped by the Fourth Doctor. When Omega's stellar manipulation device, which was dubbed the Hand of Omega,became the object of a struggle between two competing factions of Daleks, the Seventh Doctor was able to defeat them by reprogramming the device. When used by Davros to try to duplicate Omega's results, the device caused the apparent destruction of Skaro and all the Daleks, though they would return.

Another attempt to by the Master to defeat the Doctor, now the Eighth Doctor, revealed a new function for the Eye. No longer just a power source for Gallifrey, an artifact referred to as the Eye of Harmony was part of the TARDIS and appeared to be the TARDIS's primary power source. It also appeared that leaving the TARDIS' Eye open for too long would result in spacetime distortion as the space around the Eye was exposed to the gravitational effects of the singularity. However, having the Eye of Harmony on board the TARDIS contradicted the idea that the Eye itself was on Gallifrey. The name however, mearly applied to a remote link to the actual Eye that powered the craft possibly in the same way the Time Lords transmitted energy from Gallifrey to the TARDIS During an attack by Omega that was draining power from the Time Lords.

The Ninth Doctor revealed that Gallifrey and the Time Lords had been destroyed in a Time War. This would seem to mean that the Eye of Harmony was also destroyed. He later stated that the TARDIS needed to "refuel" by absorbing energy from a "scar" left by a closed spacetime rift, implying that it is not powered entirely by the Eye at this time. The Tenth Doctor makes an reference to the Eye's creation when trappped on a planet near the entrance to a black hole when he states, "My people practically invented black holes. Well... in fact they did."

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