Validium

Validium was a Gallifreyan living metal which could think for itself and transform itself. It also had great destructive potential.

During an unknown incarnation, the Doctor allowed a piece of Validium to fall to Earth and then allowed Lady Peinforte to construct a statue of herself out of it making it the Nemesis statue. The Doctor launched Nemesis into space in a rocket-powered asteroid, though the bow and arrows 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, he returned for it as in approached Earth again in 1988.

The Doctor, who could make Nemesis follow his instructions, bade the Nemesis to destroy the fleet of Cyberman ships in Earth orbit and then sent it back out into space. (DW: Silver Nemesis)