Key Zero: Disappearances:
An undisclosed number of angels go missing from Heaven every year. A lesser percent end up on Earth and live among humans. The larger cut ends up in the Fallen City. The angels in the Order try to figure out how this keeps happening. For years, they couldn't come up with the right answer or how to stop it.
One of the key problems is that once an angel is lost, there is no way to contact them or track their every move. Earth has spotty connections due to the mix of demons and angels shuffled among the humans. The Fallen City is the biggest dead zone that the Order has encountered. Once an angel ends up there, all hope of getting them back out goes up in flames. Nobody really controls the Fallen City, therefore the Order and the rest of Heaven can't decide on how to administrate the correct divine punishment and get back their lost angels. In the end, the multiple missing cases wind up piled high in the back office only to be glanced over once in a golden moon.
The Order has another missing angel case on their hands. Allen Walker has been lost from Heaven since back in the last spring. The heads of the Order feared the worst. The only problem they had was trying to retrieve him. They couldn't even confirm if he was still alive. Even still, some of the other angels insisted that they still search for and bring him home.
Meanwhile, the Fallen City gave way to a dusty summer that year.