If the ravens leave the Tower, the Kingdom will fall.

The legend is older than the Service and older than most of the kings whose ravens they are said to guard. It begins in the reign of King Charles II, who was warned by his court astronomer that if the ravens ever abandoned the White Tower, the Crown would fall and the kingdom with it. The King, a man not easily rattled, gave a single standing order that has never since been countermanded: so long as the ravens remain, the Tower stands; so long as the Tower stands, the Kingdom holds. Six birds, minimum, at all times. By royal command. In perpetuity.

The roots run deeper still. Long before Charles, before the Normans, before the Tower itself, the ravens belonged to the old Welsh god Brân the Blessed, whose name means raven, said to have ordered his own severed head buried beneath the White Hill of London to ward the island from invasion. For as long as Brân's head lay where it lay, no foreign army would ever take Britain. The ravens, the legend tells us, are what remains of him. They are the watch he set, and they have not yet stood down.

The men and women who pass through Unit Z believe, without exception, in the legend of the ravens. Every one of them keeps a count of the Tower birds in the back of their head, and every one of them sleeps a little less easily on the rare nights the count comes up short.

They are the oldest active asset on the British register. They have outlived governments, empires, and numerous Directors of S.E.S. They will, in all likelihood, outlive this one too.