Pyrovile

Pyroviles are a sapient species of mineral-based lifeforms originally hailing from the planet Pyrovilia. The Pyroviles are about thee meters tall humanoids made out of a rough rock crust with a molten magma interior, as visible from the orifices of their faces; and capable of shooting out fire from their mouths. They need a hot dry environment and are immediately killed by liquid water. Despite this, they are known to survive as minuscule particles which, when inhaled by Humans, trigger a biological transformation which slowly turns the Human body into a new Pyrovile.

In the time of the Roman empire on Earth, a Pyrovile craft fleeing from their recently destroyed planet crashed on Earth into Mount Vesuvius and disintegrated along with its crew. The still living Pyrovile particles were mixed within the smoke exhaled by the dormant volcano and ended up inhaled by some of the citizens of Pompeii, who started to undergo the slow transformation into rock creatures. Because the Pyroviles gave them the ability to predict the future, the affected ones formed an organized society bent on helping the aliens conquer all humanity, after which they planned to tamper with the Earth ecology to make it more akin to their needs (eliminating the water bodies and increasing the temperature, mostly). The Doctor prevented this by triggering the eruption of Vesuvius, resulting in the destruction of the Pyroviles but also of the city of Pompeii.