Sleeper, Awake!
When their Father cast them from Heaven, his Angels lost themselves in their grief. In 1424, they haunted the skies of citadels, as winged mercenaries turned animal, seeking bloodshed for a slight they no longer remembered. From Ardus, the central city of the kingdom, groups of soldiers took to the Roads to provide crucial supplies and transport amongst citadels. When Castiel, the last remaining Angel equipped with any sense of humanity, happened upon an attack to a Slayer's party on the Road, he rescued the only survivor - Dean Winchester - and spared him from certain death. Under starry skies and the shadowy treetops of Ardus' forest, the pair breached the boundaries of their relative humanity and divinity, discerning first friendship and then more, until, driven by love, Castiel was imprisoned by the Empress Lilith for the crime of animalism.
When the door to the tomb is opened, Dean is returned to him. Only Dean no longer knows him, and the year is 2013.