The Imperial Response
Heroic Defence
The traditional Imperial response to alien attack is based on entrusting each system to its Governor. Because of the time lags implicit in warp communication and travel, each system must be able to defend itself and the resources of an entire world are not to be sniffed at. This type of campaign is really the flipside to the Red Harvest campaign, with the emphasis placed on the Necrons' victims. A campaign featuring several Imperial players fighting as a team against a Necron force commanded by the campaign designer can make an interesting change of pace. The Necrons can be played more for drama than out-and-out efficiency, and missions can be created that are deliberately unbalanced.
Inquisitorial Investigation
The Ordo Xenos will inevitably take a major interest in the Necrons and the C'tan. This provides a great opportunity for a cross-system campaign using the Inquisitor rules. Cross-system campaigns work best if you organise them in phases during which you stick to one game system and then take the consequences into account in the next phase. An Inquisitor campaign, particularly one involving bands which include Tech-priests, can flesh out the background for a Warhammer 40,000 campaign very effectively. As an example, two bands could be seeking a Necron tomb, one band containing a Tech-priest who regards the Necrons as sacred living machines, the other a Tech-priest who sees them as abominations. With one outcome the tomb might be raided by Space Marines, with another the Necrons will be awoken to attack an unwary Imperial world.