In this world, there was a huge war against the demons a few centuries ago... this collapsed the empires at the time and created the point of light setting (an idea that I had in concept, if not in name, long before I read 4e :P). That war shook the world and even gods died.
Which is the key point. Now a powerful priest believes that if he can start the war again, the gods will have no choice but to defend the world once more, and that one is certain to fall... and he'll be ready to take it's power and ascend goodhood himself.
He's not overly concerned about the fact that the Demon may win and destroy the world, he's confident that once he is a god, he'll be able to send them back.
The Priest has several followers who will follow him to their death - they believe they will be made his Exarch when he ascends.
While the players will eventually face him, it isn't mean to happen before well into the Paragon - if not Epic - Tier...
So how does this backstory affect them?
Well, part of the ritual needed to re-open the demon gate is the blood of heroes. The PCs, a bunch of adventurers, will eventually earn a name for themselves... and the BBEG's follower will start to 'challenge them'. They will do their best to kill the PCs, using all their ressource... if they kill them, then obviously the PCs weren't mean to be the heroes. If they are defeated... then the PCs are one step closer to reaching the 'hero' level needed.
All the while, this builds them up to face the BBEG.
Now the problem I have here, is connecting the dots.
I figured I'd use Kobold Hall as a first game, since it's a good module for me and the players to get used to the rules... and killing a dragon is a pretty good start to 'hero-dom' as well as certain to make the BBEG's follower take notice of them.
The first Follower to test them will be a Cleric as well, he's going to have undead at his command. He's also got contact with a local tribe of Hogoblin... he'll convince the hogoblin that now is a good time to strike the town the PCs are in, and after the slaughter, he'll use the fresh corpse to build an army of undead to strike at the pc once more.
So this my idea for the first few game sessions.
After that, it's pretty open ended, but I'll have time to think of more by then.
I also don't want the games to be non-stop fighting... I already have a city siege planned, followed by an undead 'invasion' - they'll be plenty of hacking to be had. I will throw in a lot of social interaction with the town's npc, who feel honored/threatened/whatnot to have dragon slayers in town, and who want to manipulate them toward solving their problems/eliminating rivals/etc... or who just don't want heroes around and want to get rid of them, etc.
By the time the necromancer show up, they should have several level done, and have more or less outgrown the town.
Any suggestion/idea as to what to do after the Necromancer or what kind of challenge to throw at them while in the town, or even how to handle the Hogoblin Siege and Undead Invasion (especially to make them seem different) are welcome. Brainstorm away
Could use the undead attack as a means to get them asking questions, researching and traveling to nearby villages/towns to inquire about the undead.
Create a few small adventures using Dirtyfrank's idea of attacks to outlying villages. I would make the undead attacks a bit mysterious, whole farms abandoned, the farmers were captured or killed to become soldiers in the undead Army. You can also have the cleic create undead from the bodies of the Kobolds and any other foes the PCs killed in Kobold Hall. Attacks by the Hobgoblins should be more obvious, survivors tell of the well armed organized attacks. Maybe have the PCs attack a farm the Hobgoblins have occupied.
Add pressure from the locals on the PCs, as Heros they should be solving the problem of the attacks.
As to the siege of the town, have it start while the PCs are away chasing down a small group of Hobgoblin raiders. When they arrive they will have to find a way to fight or sneak their way into town. The attacks should be waves of undead attacking with small groups of Hobgoblins waiting to exploit any breaches in the defense. Play up the horror of the townsfolk of having ancient dead bodies and the bodies of friends or relatives as part of the undead horde.
Make sure the PC's need to return to town a couple times before they can meet the Undead Summoner. First time, one NPC they met seems to be acting alittle strangely. Second time it's more of them, some giving them a wider berth than normal. By the third time and onward the conversion should be well under way.
After they kill the Undead Summoner, have them return and that night as they rest the Inn is set on fire. Now the PC's need to escape the flames, with a ring of murderous cult members, recent conversions, waiting for them outside.
Now all of a sudden a town they liked, and people they liked are their enemies. And there's no way to tell who is with them, and who is secretly against them. Easily could become a great adventure of cat and mouse across a small city.
And of course, towards the end, as they discover the converters identity - have them corner him and he calls on a ally to save him. This could be one of your big mini-bosses. The kind that fight like a demon, but when the Converter is killed, or the mini-boss beaten badly enough he escapes... leaving the PC's now to hunt him down. Maybe have him kill a good NPC, one that's saved them from the earlier witch hunt.