The Warfreekz were an advanced race whose home was half a billion light-years from Earth. Seeing warfare as a form of art, they specialised in "conflict management" as part of their search for the perfect slaughter.
In 1914, the Warfreekz sent one of their surveillance devices to Earth so they could observe the battles of World War I. Sometimes the soldiers saw the device and rumours began to spread of an angel of death coming to take the dead to Heaven. The Tenth Doctor and Rose Tyler arrived, and after discovering what was going on, the Doctor reminded the Warfreekz that even watching wars was not painless and sent them a feedback pulse which hurt their ears. After this, the Warfreekz withdrew from Earth. (COMIC: Warfreekz!)