Verulan

The Verulans were an alien race that tried to conquer the Earth in the early 21st century. They were going to use a weapon which could paralyse lifeforms over a large area to do this. The Fourth Doctor managed to reflect the signal back on the Verulans, destroying them. (PROSE: The Last Broadcast)

However, a parasite known as the Darkness that was attached to the Verulan ship fell to Earth when the ship was destroyed. The Sixth Doctor dealt with this aftermath of the attempted Verulan invasion. (PROSE: The Terror of the Darkness)