In an alternate timeline, the Pyroviles conquered Earth in 79 A.D. after Pyrovillia mysteriously disappeared, in the process altering a fixed point in time. It was averted when the Tenth Doctor and Donna Noble caused the eruption of Mount Vesuvius, fixing the timeline and destroying the Pyroviles.
History
Disappearance of Pyrovillia
When their homeworld was stolen by Davros, (TV: The Stolen Earth [+]Russell T Davies, Doctor Who series 4 (BBC One, 2008).) 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 were able to tell the future accurately from the time rift which formed around Pompeii. (TV: The Fires of Pompeii [+]James Moran, Doctor Who series 4 (BBC One, 2008).)
Arrival of the Doctor
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. As a result, the volcano would never erupt, creating an alternate timeline and changing a fixed point in time in process.
During this time, the Tenth Doctor arrived with Donna Noble during a failed attempt to take his companion to Rome. While initially refusing to get involved as the eruption of Vesuvius was a fixed point in time, the Doctor became interested after learning that the soothsayers, despite having accurate powers of prophecy, couldn't see the upcoming eruption. The Doctor's subsequent investigation led to his discovery of the Pyroviles and their plot. (TV: The Fires of Pompeii [+]James Moran, Doctor Who series 4 (BBC One, 2008).)
Negation
The Doctor and Donna became trapped in the Pyrovile pod at the heart of Vesuvius where the Pyrovile control circuitry had been set up. The Doctor realised that the Pyroviles were draining the volcano's power for their converter which was why the soothsayers couldn't see the eruption: in this future that the Pyroviles were creating, it would never erupt.
The Doctor was devestated to realise that the only way to save the world from the Pyroviles would be to cause the Eruption of Mount Vesuvius himself, killing 20,000 people to save the world, meaning that the destruction of Pompeii wouldn't just be history, but the Doctor himself. Supporting the Doctor's decision, Donna pushed the control lever with him, causing the Vesuvius to erupt, destroying the Pyroviles and ejecting the pod carrying the Doctor and Donna to safety.
The moment that the Doctor and Donna triggered the eruption, the soothsayers could detect that the future was changing and there was only death for everyone. However, they also sensed that it was the correct future. Donna managed to convince the Doctor to rescue Lobus Caecilius and his family from the eruption and his daughter Evelina reported that her visions had gone. The Doctor explained that the explosion generated by Vesuvius' eruption was so powerful that it opened a rift in time for just a second which echoed back into the Pyrovillian alternative and allowed those with psychic powers granted by the Pyroviles to see into the future. (TV: The Fires of Pompeii [+]James Moran, Doctor Who series 4 (BBC One, 2008).)