I would start with the idea that the demon lord needs a certain number of things to happen (or to have possession of) by a certain date (# based on how long you want the campaign to be). I find it best for the PC's to stumble onto this (maybe one of the demon lord's mortal minions is an enemy of one of the PC's). The hardest part of something like this is that you want the demon lord to "win" enough that he/she/it shows up and the PC's fight he/she/it before the portal opens while balancing not wanting the PC's to lose constantly before the big fight. I tend to go with "you stopped him from getting the doodad that would open the portal today, but his other group of minions got the doodad that would open the portal next week." Also, unless the party is going to be level 16 or above or you are running a 12 PC party with twice their actual carrying capacity of magic weapons, you should slip some kind of magguffin for the PC's to use in there too.
