The Night Travellers stole people's breath. After travelling circuses fell out of fashion when film and cinema became popular, all that remained of the Night Travellers were lost and scattered old films of them, which apparently also contained the Night Travellers' power and/or their personalities inside.
When the Electro reopened and one of these films was shown, the Ghostmaker and Pearl were freed and set about taking victims' breaths. They attempted to free the other Travellers and succeeded. They were defeated by being filmed once again, recapturing them on celluloid, followed by Torchwood Three exposing the film to the light and destroying them. Jack Harkness claimed, however, that they could come back if there were other old film canisters containing them. It was indicated that Jack's fears were true, as when an old film reel was opened after a father and son purchased it at a car-boot sale, Jack briefly heard a sliver of the Night Travellers' music; indicating that they were on it. (TV: From Out of the Rain)