The Armidians stood over seven feet tall, had disproportionate features and translucent skin. They had a telepathic affinity with tharocite, and used the Tharon stone to manipulate the forces in all igneous rocks.
Circa 2000 BC, an Armidian exploratory research group including Garlin Munn came to Scotland on Earth to find out about Earth's inhabitants, minerals and fauna. They set up a collection of data collection reservoirs that blended in with the area's stone circles, each one containing a data collection stone that passively absorbed data for years. The party of ten was ambushed by tribal human warriors. Six of them were killed, and the remaining four were left for dead. They were placed in healing capsules underground along with the Tharon stone.
Tharos, along with the Valerian system, were destroyed in an interplanetary conflict around 1000 BC, leaving the scientists on Earth the last of their kind.
After being awakened by roadworks in the 1970s, the last Armidians recovered the Tharon stone, and shot down an RAF fighter jet. The jet was taken apart and examined in a fishing shed in Portsoy surrounded by a perception barrier. They forcibly questioned the pilot Frank Parry about the planet's weapons capabilities, but this process damaged his brain. Mike Yates was then questioned by them. The Third Doctor informed them that Tharos had been destroyed. Yates broke free and Sergeant Benton opened fire on the Armidians as they escaped, killing of them. To protect the last of their kind, Garlin decided to kill the aggressor humans on Earth with the Tharon stone and make Earth the new home for the Armidians. The two Armidians at relay points were killed by UNIT teams led by Benton and Brigadier Lethbridge-Stewart, and Yates and his team earthed the Tharon stone with some wire fencing, rendering it powerless. The Doctor survived a murder attempt by Garlin, but was unable to save him. (AUDIO: Vengeance of the Stones)