The Traveler once went to the Fallen before us to advance them so they could defend it. Sadly they weren't strong enough and were abandoned, becoming the "Fallen". They didn't have the energy of light in them anymore but the servitors were able to harness energy from various things which the fallen would use to survive, and that energy is called ether. All fallen houses were grouped together and controlled by The House of Judgement before the traveler left them. After hundreds of years they tracked the traveler to earth with the darkness and seek to take it back.