Xiuhcoatl, which translates from Aztec to mean Torquoise Serpent, was the name given to an ancient Exxilon weapon abandoned on Earth.
It was described by The Doctor as the most powerful and precise even he had evenseen. It was capable of fusing two molecules of hydrogen together or turna pool of water into a nuclear bomb. In addition, it was capable of hollowing out a planet or carve a name on a tree.
The weapon was discovered by Huitzilin who used it to vaporize a mile-wide stretch of forest and the four hundred men that were hiding within it. (NA: The Left-Handed Hummingbird)
