Deep within countless worlds across the universe, an entity waits. Cold. Calculating. Impervious to the ware of time. Eventually, these beings will awake and rise from their tombs to reclaim the surface, and eventually the galaxy. These beings are known as the Necrons, an ancient race that sold their souls and minds for eternal bodies that could survive almost anything. It has been many eons since they last were active, and their time might be coming again. With their minds that have only grown wiser all this time and their everlasting hate for the living, their Overlords are simply waiting for the proper time to reclaim their lost thrones.
Though there are currently no active tomb worlds recorded, the threat of a Necron attack is one that the Imperium is always worried about. Even though the citizens, and most player characters, will not know of or even have heard of the race of mechanical skeletons, this is kept a public post to remind players that a big threat can happen at any time.