Validium was created as the ultimate defence for Gallifrey by Rassilon and Omega and had great destructive potential. It was living metal that could think for itself and was capable of speech as well. When destroyed, it could reform itself. (TV: Silver Nemesis)
When Rassilon and Omega found that the validium's sentience made it too difficult to control, they locked it away in an impenetrable vault where it was casually stolen. (PROSE: A Brief History of Time Lords) When Rassilon was building the Type 1 TARDISes, validium was used in their fluid links. (COMIC: The Lost Dimension)
It should never have left Gallifrey, but some of it did. A piece of validium fell to Earth and was found by the Lady Peinforte, who moulded it into a statue of herself holding a bow and arrow and called it Nemesis. The Doctor then launched the Nemesis into space on the rocket-sled, though the bow and arrow were kept on Earth. Its orbit had brought it near Earth every 25 years, each time causing disasters by remote influence. In his seventh incarnation, the Doctor arrived on Earth as it approached again in 1988.