Lucius visited the Caecilius family the day before the eruption of Mount Vesuvius to get part of a marble energy converter designed by the Pyroviles from Lobus Caecilius. Under the direction of the Pyroviles, he collected the pieces to build the converter inside the heart of Mount Vesuvius. It would convert the population of the world into more Pyroviles.
The Tenth Doctor and Caecilius' son Quintus found out he had inhaled Pyrovile dust. This was slowly turning him into stone, with his right arm having to be covered by a toga as it was stone. The Doctor broke his arm off and escaped with Quintus. Later the High Priestess ordered him to advance the prophecy. He presented the energy converter to the Pyroviles in Mount Vesuvius. The Doctor and Donna caused the eruption of Mount Vesuvius, killing the Pyroviles and Lucius inside it, along with the converter and the rest of Pompeii.
While trying to figure out the stolen planets, Donna remembered what Lucius had said about Pyrovillia and suggested it as another stolen planet. Donna remembering and repeating what Lucius said helped the Doctor to realize that the planets were being stolen from across time, not just space. (TV: The Stolen Earth)