He is played by Phil Davis, who also portrayed Jeff Hope in the Sherlock episode "A Study in Pink".
He first seen arriving at the home of the Caecilius family the day before Mount Vesuvius erupted to get a marble energy converter designed by the Pyroviles from Lucius Caecilius Iundus. Under the directions of the Pyroviles, Lucius collected the pieces to build the converter inside the heart of Mount Vesuvius to convert the world's population into other Pyroviles.
The Doctor and Caecilius's son Quintus found out Lucius had breathed in the dust of Mount Vesuvius which was actually the remains of the Pyroviles which was slowly turning him to stone. The Doctor broke his arm off and escaped with Quintus. Later the High Priestess of the Sibylline Sisterhood ordered him to advance the prophecy and he later presented the energy converter to the Pyroviles in Mount Vesuvius. He was killed along with the Pyrovilles and the other residents of Pompeii (with the exception of the Caecilius family) when the Doctor and his companion Donna Noble caused Mount Vesuvius's eruption.