The Necrons and the Imperium
The Necron Threat
The Necrons present an inimical threat to the Imperium but it is not the same type of threat as a major Ork Waaagh! or the advance of a Tyranid hive fleet as the Necrons have no interest in gaining territory or fighting wars on huge battlefronts. Instead they strike without warning and withdraw once they have achieved their objective. Their approach is therefore idiosyncratic and may, for example, be more concerned with causing terror than defeating armies.
The Red Harvest
The C'tan are energy vampires who savour the unique essence of sentient creatures. As such they do not regard their attacks on the Imperium as warfare but as a harvest of lesser beings. When a world is harvested there will be major battles as the Necron Warriors descend, for which the standard Warhammer 40,000 missions can be used, but there are also possibilities for more unusual games. For example, a mission could feature a group of retreating Imperial Guardsmen being chased down by a pursuing pack of Necron hunters, or an advance into a ghost town where Flayed Ones lurk amongst the bodies of their victims. Horror movies provide a great source of inspiration for this type of mission and the Necrons really work well in this type of game, with their refusal to stay dead and a terrifying arsenal of weapons.
Infiltration
The C'tan and Necrons do not want their enemies to have the opportunity to develop new weapons to use against them. This opens the possibility of campaigns where the objective is a key Imperial research station or a particular individual. Such bases will be well-defended and will require a raid rather than an all-out attack by the Necrons. A campaign based on this premise could use the Cityfight rules to cover fighting within the complex itself and should be set up so that time is on the side of the defenders with the prospect of reinforcements arriving.
Graven Images
The Imperium is a vast collection of disparate worlds. Often the Imperial Creed is the only unifying factor. In some places even that is lacking and here the people are vulnerable to falling under the sway of the C'tan. The star-vampires delight in being worshipped as gods and the vast power that they wield is a convincing argument to a primitive culture. C'tan domination of such a world might be countered by Imperial agents or perhaps Space Marines who periodically recruit or train there. Naturally the locals might be induced to fight to defend their new god.