A lightwanderer was a crystal-like lifeform.
A lightwanderer was found by Nazis on Earth during World War II. They captured it and used it to place a barrier around the English village of Pax Lucis. They also planned to use it in a machine that would destroy England, and in Gruber's opinion, the entire planet, ignoring the Seventh Doctor's warnings.
After Luther was relieved of command and shot, he fell on the machine, destroying it and freeing the lightwanderer. As the lightwanderer escaped, it killed the remaining Nazis. (PROSE: Special Weapons)