Rutans

Green, blob-like aliens, engaged in an endless space war with the Sontarans.   Rutans likes the cold, had natural affinity for electricity, and were able to shift their shapes.  A Rutan scout became stranded on the Island of Fang Rock, and after killing Reuben, planned to use Earth as a strategic point in the war with the Sontarans.   The Rutan eventually killed the other lighthouse keepers, Ben Travers and Vince Hawkins, as well as the survivors of a shipwreck: Lord Palmerdale, Adelaide Lesage, Colonel Skinsale, and crewmember Harker.  The Fourth Doctor and Leela were finally able to destroy the Rutan with a  rock launcher.  The Doctor blew up his mother ship by using Lord Palmerdale's diamonds, and converting the lighthouse into a crude laser cannon.