Styggron developed Kraal androids. They were created through the use of a pattern analyser, which implanted the memories and intelligence of a subject into an android, which then took on the form of its subject. They were nearly indestructible, but Styggron produced weaponry capable of destroying them. The analyser could also extract information to the Kraal data banks.
The Kraals lived on a doomed world, evolving due to exposure to radiation over a dozen generations. (AUDIO: The Oseidon Adventure) Oseidon was becoming more radioactive, reaching critical levels which the Kraals could not withstand. They abducted a human astronaut, Guy Crayford, and scanned his brain. Styggron tailored his androids to look like the inhabitants of Devesham, an English village, which was also produced, along with the adjoining Space Defence Station, as a training ground on Oseidon. After testing these androids, they planned to send them to Earth with a deadly virus to infect and kill all of the humans. The Fourth Doctor used the radar dishes at the real Space Defence Station to disable the androids there and then boarded the rocket Styggron had used. Crayford confronted the Kraal about his lies, having promised that no humans would be harmed. Styggron killed him but was thrown by an android of the Fourth Doctor onto the steriliser containing the virus. He was infected but managed to fire his gun, destroying the android, before he died. (TV: The Android Invasion)
With the help of the Master, the Second Kraal Army attempted to invade the Earth again, by using a wormhole that the Master generated in Derbyshire. Upon arrival on Earth, the Kraals, led by Marshal Grinmal, double-crossed the Master and imprisoned him with Leela, while they sent the Doctor to Oseidon to be interrogated by Chief Scientist Tyngworg. Grinmal allied himself with the Master's former ally, Colonel Spindleton, as they drew out their battle plans. Grinmal allowed Spindleton to take possession of Great Britain after the invasion, in exchange for knowledge to conquer the world. However, while this was going on, the Master and Leela escaped through the wormhole, and infiltrated the Kraal bunker. However, while he was in the bunker, the Master discovered he was an android duplicate, ever since he arrived in Derbyshire, and the Master had been on Oseidon all along, impersonating Tyngworg. During this, the Doctor escaped and reprogrammed the androids to destroy the invasion force. But, as the Master tried to deactivate all the androids, he discovered he was susceptible to the signal, and therefore, he had also been an android all along as well. After the destruction of the Second Kraal Army and the androids, Grinmal was the only surviving Kraal. He allied himself with Leela and the Doctor, so he could stop the Master's real plan, which involved destroying Oseidon. While trying to stop the plan, Grinmal was shot by Spindleton, but the Master's plan was foiled. (AUDIO: The Oseidon Adventure)
Behind the scenes
According to The Brilliant Book 2012, a non-valid source for in-universe articles, in 1881 they attempted to replace Prince Bertie with an android duplicate, but the Doctor[which?] and Jenny stopped them.