Eye of Harmony
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The Eye of Harmony, in the long-running British science fiction television series Doctor Who, is the name given by the Time Lords to the artificially created black hole that provides nearly inexhuastible amounts of energy to their home planet of Gallifrey. It also makes time travel possible by acting as a remote power source for their TARDIS time machines.
The Eye was originally created in the distant past by the Gallifreyan scientist Omega, a solar engineer who used the Hand of Omega, a stellar manipulation device, on a star named Qqaba in the vicinity of Gallifrey. The resulting supernova apparently killed Omega and created a black hole. Unknown to anybody for millennia, Omega was not dead, but merely trapped beyond the event horizon of the black hole in an antimatter universe (The Three Doctors).
The nucleus of the black hole was then captured by Omega's colleague Rassilon to act as a power source for time travel. This nucleus, known as the Eye of Harmony, eventually made the technology possible for the Gallifreyans to become the Time Lords and for Rassilon to become their ruler. The Eye was then placed beneath the citadel of the Time Lords on Gallifrey, known as the Panopticon. It remained there for so long that it passed into legend, and until the Doctor rediscovered it in the serial The Deadly Assassin, the Time Lords had forgotten its location, some even believing it to be mythical.
The Eye is linked to two other Gallifreyan artifacts, the Sash of Rassilon and the Great Key of Rassilon (not to be confused with the other Key, which permits access to the Matrix, the Time Lord computer network), also known as the Rod of Rassilon. The Sash is needed to safely control the Eye without being sucked into it, and the Great Key can be used to drain energy from the Eye. In The Deadly Assassin and the Doctor Who telemovie, the Master tried to utilize the Eye's energy to give himself a new set of regenerations.
Each individual TARDIS contains a remote link to the Eye that powers the craft. Confusingly, this link is also referred to as the Eye of Harmony. Leaving the TARDIS's Eye open for too long will result in spacetime distortion as the space around the Eye is exposed to the gravitational effects of the singularity. The spin-off novel The Quantum Archangel by Craig Hinton claims that all TARDISes built after a certain point, including the Type 40 the Doctor uses, have a mathematically modelled duplicate of the Eye with all its attendant features rather than a link. However, like all spin-off media, the canonicity of the novels is unclear.
In the 2005 series episode The End of the World, the Doctor reveals that Gallifrey and the Time Lords have been destroyed in a war. It is unclear if this means that the Eye of Harmony has been destroyed, but in the episode Boom Town, the TARDIS needed to "refuel" by absorbing energy from a "scar" left by a closed spacetime rift, implying that it is no longer powered by the Eye.