Xiuhcoatl, which translated from Aztec as Turquoise Serpent, was the name given to an ancient Exxilon weapon abandoned on Earth.

The Seventh Doctor called it the most powerful and precise weapon he had ever seen. It was capable of fusing two molecules of hydrogen together or turning a pool of water into a nuclear bomb. It was capable of hollowing out a planet or carving a name on a tree.

The weapon was discovered by Huitzilin, who used it to vaporise a mile-wide stretch of forest and the four hundred men hiding within it. (PROSE: The Left-Handed Hummingbird)