Hidden, mist-shrouded planet once inhabited by a highly advanced race of silicon-based beings. The Exxilon visited Earth and were mistaken for space gods by the Incas. They created the City, a computer-controlled complex that thought itself so perfect, that it cast out its creators, who then reverted to savagery. A group of Exxilon, led by Bellal, stopped worshipping the City. They helped the Third Doctor fight the Daleks and, ultimately, destroy the City. The mineral parrinium, the only antidote to a space plague, was found only on Exxilon (XXX).