Help me build my first adventure

[Chaosvii7]

I'm trying to devise my first campaign and I'm trying to take the process slowly to make sure I hit everything on the way. I've a pretty good idea on the whole overarching story of the campaign - a typical good vs. evil fight with the buildup to an eventual war towards the end - and I have enough details to start rolling with it and building the world around the players as they explore it, I think.

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The players are starting in an empire, coming into a city for different reasons. For the Fighter, he's heading to this city because he seeks recompense for his bar, which was burned down in the night. The Rogue is heading to the city to talk to their former employer, the head of the local thieves' guild, as this player got that person's base destroyed(but the character is so gullible that they don't see any harm in it). And the Wizard is "on vacation" from his Mage College, and is so ashamed at being kicked out due to poor academic performance that he's deciding to run away from his family. The Ranger is technically a vigilante, but he's also considered a fib or tall tale (like Year One Batman was.)

Each one is coming from one of the different roads into the town, and each is traveling with a caravan. What they don't know(besides each other, initially) is that these three caravans are each disguises for a cult that is infiltrating the city to begin operations there.

I'm going to run each character's introduction one by one before bringing everything together, and I have a good idea how; The city is going to offer The Fighter a job on the graveyard shift with the city guard, The Rogue is going to get a task from the thieves' guild, and The Wizard is the target(the Wizard is mistaken for being a part of the cult that came into town, this thieves' guild on the up and up that a cult will just draw more attention to the criminal underworld of the area.)
Basically, to get the party together, The Ranger will save The Wizard who's being hunted by The Rogue who's being actively pursued by The Fighter. And then the cult will attack them as a group and they'll find themselves sinking into conflict with them, becoming allies before forming a pretty lasting bond.

The only question I have is, how do you think I should play up the cult? They aren't just moving in to get creamed by the PCs. They can do any sort of logical cult stuff, really.

To that effect, what should the cult be of? A death god? A demon? Or are they a misunderstood sect of a different, unconventional god(like the sun?) They could kidnap people, summon something, raise the undead, there's no limits set for them technically. They will be in the sewers, which might limit what kinds of encounters I can toss out, but otherwise does anybody have any suggestions on what I can do with the cult?

Also, a fair critique of this as a starting adventure would be nice. I'm also willing to listen to any sort of recommendations to tweak what I've already got in place. I want to offer a good and intriguing story to my players, and I hope this is a step in the right direction.

[NickChaisson]

So far that all looks good except that it all seems based on PC choice. If the players decide to not chase each other than your going to have to improvise.

Make the Cult a Cult of the Machine. Kind of like Cybermen from Doctor Who. They turn people into robots or monstrous cyborgs. Open up planar portals and change the surrounding area into a more mechanical one. They view themselves as saviors from the Machine god, coming to relieve you poor souls from your meat sacks and ascend into a higher plane as a robot. You could even upgrade them from cult status and make them an official church in your setting. Sure they spout crazy, but they're the best craftsmen around/have advanced technology so people tend to leave them alone. Hopefully that gives you some inspiration ^_^

[Fire Lord Pi]

Two things.

This is a great idea. The "how the the PCs met" story is laid out quite beautifully. In theory. I've played D&D enough to know, for absolute certain, that the only thing you know for absolute certain is your own plans. What actually happens is an entirely different subject, sometimes not even related. I'd be very careful if I were you to come up with some contingencies. If things go wrong (and they will), be prepared. Murphy's law and all that.

The cult has a lot of potential. I would make it of the "anti-religion" to whatever faith is dominant in the the town in question. The cult is attempting to assassinate church leaders and authorities who might stop the spread of their new religion. This could lead to lots of interesting encounters, in which the PC's must deal with and watch two powers battling for the support and faith of the common people. If you go this route, you would need to really develop the customs and contrasts of these two religions.

As for your finale, this would be perfect. Fights slowly break out between the cult (and their converts) and the church (and the townspeople). Things escalate until full on riots break out, culminating in a battle to control the central defensive structure (castle, fort, whatever). Hope this helps!