The situation you describe essentially not only makes PC choice irrelevant but also makes success or failure of thier efforts meaningless.
It really hasn't, actually, but I can understand why you might see things that way.
I have a lengthy article I sometimes post at times like this about DM perspective and Player perspective, and how I think that what you're doing is looking at DM decisions from a Player perspective, but meh, I don't feel like writing a book today.
S'mon said:
Technically this is what Ron Edwards calls 'Illusionism' rather than railroading - creating the illusion that PC choice matters. It's less irritating to players than railroading, but if players find out about it they'll likely not be happy.
This is definitely true.
S'mon said:
Personally I would not do this - the number of zombies the Necromancer has should be determined by PC success/failure; if he doesn't have enough he'll go do something else, maybe go looking for a necromantic artifact and come back later. Alternatively, if the town does get attacked, there may be more or less zombies, making a harder or easier fight, and more or less dead townsfolk.
But the very real danger of the approach that you've chosen is that your campaign might suck. You're going to be trying to balance three concerns- that the "natural" level of zombies if the PCs fail is high enough to be scary but not kill off the party, that the modified level of zombies if the PCs succeed is still high enough to make the fight a valid climax to your adventure, and that the difference between the "natural" level and the modified level is large enough to make sure that the PCs feel that their efforts in sabotaging the necromancer were worthwhile.
Good luck with all of that.
Far better, in my opinion, to handle the matter through roleplaying. Now, in reading the following, keep in mind that this ONLY matters from the perspective of the players in the actual game, NOT the perspective of a hypothetical third person omniscient character who knows what would have happened if things were different.
The PCs attempt to sabotage a necromancer who is trying to raise zombies to attack a town:
Option 1: The PCs succeed. Had they not, there would have been so many zombies that the town would have been overrun with or without their presence. Fortunately, since they sabotaged the necromancer successfully, there are fewer zombies. The fight is roleplayed as an attack by a desparate necromancer who knows that unless he kills the PCs immediately and recovers what they stole from him (or whatever), all will be lost. Therefore, he's throwing everything he has into this climactic battle, and even risking his own life. His dialogue, and the banter between the villain and the PCs, is affected by this, with the necromancer swearing vengeance and crowing about how, once he's killed the PCs, he'll use their bones to construct the throne on which he'll reign over his necromantic empire. The overall encounter level is that of the PCs +3, and victory will most likely result in the necromancer's death.
Option 2: The PCs fail. The necromancer has his full forces at his command. Had the PCs succeeded, there would be fewer, but they did not. Buoyed with overconfidence, the necromancer launches a full scale assault on the town, certain that his enlarged forces will easily crush the PCs. He is so confident in victory that he personally enters battle, so that he can see the faces of his nemeses as they die. His dialogue, and the banter between the villain and the PCs is affected by this, with the necromancer sneering about the inevitability of his victory, and mocking the PCs for their failure. The overall encounter level is that of the PCs +3, but because the necromancer entered the fray personally, victory will most likely result in his death.
There are two reasons this does not, to me, count as railroading.
1. The journey is as important as the destination. The PC's actions had an effect- they changed the roleplaying of the final encounter. ROLEPLAYING IS A REAL PART OF THE GAME. Having your actions affect it counts.
2. From the PCs perspective, their actions DID have an effect. The effect is only an illusion in the same sense that the entire encounter is an illusion. Each possible path includes a way in which the actions of the PCs mattered- either the fight would have been impossible, and is now merely very hard, or, the fight is very hard, but with success, would have been easy. The fact that two possible paths exist doesn't matter, because the PCs only tread one at a time.
Objections which begin with the assumption that there is somehow a "real" necromancer with a "real" number of zombies that is either changed or not changed by the actions of the PCs are, well, not very strong objections. Because none of those assumptions are true. There is no real necromancer. There is no real number of zombies.