P is for... Pyroviles

Huge rock-based creatures with a molten magma core, who formed a bridgehead in the Campania region during the height of the Roman Empire, following the loss of their home planet Pyrovilia. They could kill with an incinerating blast from their mouths.

Their intention was to boil away the Earth's oceans and make the planet more habitable for them as a new home. They were susceptible to the cold water, and sufficient quantities could cause their exoskeleton to solidify and shatter.

On first arriving on Earth, they crash-landed their escape capsule and were reduced to dust.

The earthquake of 62AD in the Pompeii area triggered their awakening.

Their dust could infect human beings, causing them to slowly turn to stone themselves after first becoming their mental servants. They had the ability to psychically affect certain individuals with latent abilities in this area. Through a human agent named Lucius Petrus Dextrus who was a powerful local official in the city of Pompeii, they commissioned rock-based circuitry which would help them harness the energies of Mount Vesuvius in order to further their plans. 

The High Priestess of the Sibylline Sisterhood was also infected with their rock dust, and was now almost fully composed of stone.

In order to stop the Pyroviles, the Doctor was forced to trigger the devastating eruption of the volcano in 79AD, knowing it would kill thousands of men, women and children - their deaths being the price to pay to save the entire planet. This was a fixed point in time, which he could not alter.

It would later transpire that Pyrovilia had been taken by Davros to power his Reality Bomb, and the Doctor and Donna Noble were able to return it to its proper place and time.

Appearances: The Fires of Pompeii (2008).

• The design of the adult Pyroviles was based on that of a Roman soldier, whilst that of the Sisterhood priestess was based on the plaster casts of the victims of the volcanic eruption which destroyed Pompeii and Herculaneum.
• If Pyrovilia was put back where it came from, then these Pyroviles would never have fled from it and so come to Earth and tried to conquer it - so there would have been no reason for the Doctor to deliberately trigger the eruption of Vesuvius and destroy Pompeii. As it is historic fact, however, this must surely mean that it was simply a natural disaster.