The Diadem was a mind parasite which seemed to be a tiara inhabited by an evil intelligence. It was implied to be similar to the Keller Machine mind parasite. The Diadem was first seen on Pakha, where it possessed a Pakhar forager named Vor'r'na and plunged the native inhabitants into centuries of war with each other. However, there were some pacifists who attempted to change their society. The Third Doctor and his companion, Jo Grant, arrived on the planet and defeated the Diadem when the mad Vor'r'na fell into a bottomless chasm with the diadem. The Doctor, however, doubted it was completely destroyed.