The Arrival was an event that the Governors were preparing for under the guidance of a Weeping Angel. It was depicted in a prophetic tablet on the wall of Room M17 of Coal Hill Academy. (TV: The Lost)
Behind the scenes
After the cancellation of Class, the events of the Arrival were never depicted. Writer Patrick Ness mentioned on Twitter that a Weeping Angel civil war would have been included, as would a visit to their home world.