Strang was a Sontaran whose genetic makeup comprised an entire Sontaran clone batch.
Due to his exposure to a certain compound, his body was altered without any alteration of his appearance or biological functions. He gained an indestructible and impenetrable anatomy. His skin was impenetrable making it a virtually indestructible and resilient armour. Despite its virtually limitless resilience and indestructible structure of his tissues, his skin did not lose any elasticity or sense of touch. Unlike all Sontarans, Strang could not be harmed via his probic vent. (AUDIO: The King of Sontar)
After his death at the hands of the Fourth Doctor and Leela, a human version of Strang created from Leela's memories named Linus Strang appears in a simulated world when she is plugged into the computer, Sisyphus. In it, he was a member of the Ministry of Defence who attempted to convince Leela that her life was nothing but false memories created by a drug called Sisyphus, which the Ministry of Defence funded. (AUDIO: The Crowmarsh Experiment)