Balance and the D20 Night Watch System Edit
As a rule, when running a D20 Night Watch campaign, it's best not to allow players access to the Base and Advanced classes presented in the d20 modern system. This is for balancing reasons, as the classes contained in the Night Watch system are far more powerful than those normally found in the D20 Modern system. The primary reason for this is that Night Watch classes can enter the Twilight at will. Depending on how deep they go into the Twilight, a Night Watch character could be Invisible, Ethereal/Incorporeal, have an extra standard action every round, and gain a 30' boost to their movement speed. Using Magic (Or a select number of feats/class abilities), the Night Watch character could whittle down even a high level adventurer in short order.
Thus, it is best not to allow outside sources into the game. The exception to this rule, however, is the Night Watch Gestalt campaign settings. Please look to the Night Watch Gestalt Game section Below.
Staring into the Abyss Edit
Presented here are the fundamentals required for running a Night Watch Campaign when your players have decided to play Light Others. This is similar to most traditional DnD games where you play the "Good Guys," and run around stabbing the "Bad Guys." However, all Night Watch games rely on a hint of mystery and subterfuge. The players are pawns for the majority of the campaign, being used strategically by Geser (Head of the Night Watch) in a never-ending game of chess with Zabulon (Head of the Day Watch). Every move that the players make has been calculated in advance by the two master strategists with the aid of Divination magic. The only thing they haven't counted on, is the decisions you make at the climax of the story. Zabulon and Geser, for all of their foresight, can't foresee the players deciding to Destroy a mystical artifact rather than handing it over to either Watch. And this is where the players make a difference.
Starting a Night Watch campaign can be as simple as a routine patrol in which the players find out about a rogue Magician terrorizing downtown Moscow. Or it could be as complicated and convoluted as Zabulon framing the party members for the murder of an influential Dark Other, and the only way that Zabulon will release evidence proving their innocence is if they procure a powerful magical artifact from Geser's private vault.
If the PC's become powerful enough (Level 5 or so), Zabulon IS going to know about them, and have a contingency plan to deal with them. This doesn't necessarily mean that the enemies that he sends their way are going to plow right through them. Zabulon is a conservationist. He will send who he believes can get the job done with the least effect on his resources. He will not, for instance, send a Vampire 10/High Vampire 5 to deal with a few level 6 PC's. He'll most likely send a Vampire 8.