The Terileptils were encountered by the fifth Doctor when three criminals escaped from the Tinclavic Mines of the prison planet Raaga and their ship crashed in England AD 1666.
The Terileptils, aided by an android creation, intended to de-populate Earth by injecting the black rat with a plague virus. The Doctor offered to return the Terileptils to their home planet, or any planet of their choice.
They declined the offer and instead continued with their plan to wipe out the human population. They were finally defeated when one of their weapons overloaded and started a fire, starting the Fire of London. All were trapped and unable to escape the flames, and the fire destroyed their plague virus.
A phantom Terileptil was used by the Master to prevent Tegan and Nyssa from entering an inner sanctum in Time-Flight.
Additional information about Terileptils