A highly advanced civilisation of bald silicon-based humanoids with bulbous eyes and bark-like glowing skin, the Exxilon visited Earth and were mistaken for space gods in ancient Peru. They created the City, a huge complex imbued with artificial intelligence that turned on them, either killing them or casting them out. The surviving Exxilons then either reverted to stone-age savagery, worshipping the city and offering sacrifices to it; or fled the city and the savages but were persecuted by their peers and driven to the planet's lower levels. The mineral parrinium, the only antidote to the disease known as the space plague, was found only on Exxilon.
Source: Doctor Who "Death to the Daleks"