A Physical Temporal Nexus was an artefact that could give its holder its desires. It worked by bonding with its owner's mind, and selecting the parallel universe that its owner desires the most. There were, according to the Time Lords, only six in creation. One was found on Usurius, and two more were found on Kirith. Gungnir, the Spear of Destiny, was one of them, and was found on Earth. The Third Doctor and Jo Grant stopped a plan by the Master to steal it. (PROSE: The Spear of Destiny)