Pyroviles were humanoids with a rock-based form with a magma interior. They were able to shoot flames out of their mouths, which could turn a human to ashes in seconds. They could be killed by sufficient quantities of water, their internal heat snuffed out and their bodies solidifying. They towered over humans and their faces resembled the helmets of Roman soldiers. Pyroviles came in two varieties — a shorter, easily-destroyed "Foot Soldier" and more advanced, fully grown Pyroviles, which could reach over twice this height.
When disintegrated Pyroviles were breathed in by humans, these particles would slowly transform the humans into Pyroviles. The individuals would slowly turn into rock, until they were entirely stone. The Pyroviles were also able to enhance the latent psychic powers that human brains possessed (TV: The Fires of Pompeii)
At some point in their history, the Pyroviles developed advanced technology from rock, and could travel into space. When their homeworld was stolen by Davros, (TV: The Stolen Earth) a group of Pyroviles escaped in an escape pod, which crashed on Earth. The Pyroviles were obliterated in the explosion and eroded into dust in the core of Mount Vesuvius. They waited for thousands of years until they were awakened by an earthquake in 62. They influenced the soothsayers of Pompeii into creating a ventilation system, allowing the remains of the Pyroviles to be breathed in. These affected people, who were able to tell the future accurately from the time rift which formed around Pompeii.
By 79, full-grown Pyroviles had created a base beneath Vesuvius and planned to convert the power of the volcanic eruption to change all humans into Pyroviles and create a new Pyrovillia. The Tenth Doctor stopped this and knowingly caused Mount Vesuvius to erupt and destroy the Pyroviles and the nearby city of Pompeii. When asked by Donna Noble about the possibility of them surviving the eruption, he revealed that the blast would be too powerful for even them to withstand. (TV: The Fires of Pompeii)
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The Torchwood Institute learned about the Pyroviles by monitoring a 1923 British archaeological survey of Rome by Professor Livesy-Smythe. This turned up a scroll detailing the events, as chronicled by Caecilius. This unlikely tale was corroborated by the discovery of huge Pyrovile footprints and marble circuits in the Pompeii ruins. (WC: Captain Jack's Monster Files)