Take a page from an old Shadowrun Campaign and have vermin avatars making contact w/ beings on the material plane. Strange cults start appearing in cities with fly, spider, bee, and wasp themes. Strange mutations start to occur, lycanthropic spiders for example. Start the campaign in a major city and just sprinkle these things along, maybe in the midst of a low level thieves guild war or the like that the PCs get invovled in (there are pleanty of good low level city adventures out there you can run.) After a clue or two, or even a low level villan in a side quest or recovered artifact for someone who is more than they appear, start having the PCs encounter more obvious signs of the vermin avatar's influence. Monsterous vermin, the poor section of the city walling itself off and creating a giant hive etc. This would be in the low to mid levels. If the players like the way things are going and the PCs pick up on the hooks and don't do anything total stupid and unexpected (never mind, they will, just deal with it) you start introducing the vermin lord's high level henchmen at about 8th level. The vermin lord is trying to raise these various avatars to serve him, but they are warring amongst themeselves as well as causing problems for everyone else. Unless the PCs put a stop to things, a couple of the henchmen merge with the avatars and become real problems, all while opening things up for their master. Perhaps this vermin lord is actually a centuries old lich or mummy who needs to power of the avatars to be brought back to unlife. The hencmen sacrifice themselves to their master, he wakes up. Swarms happen. World dies with a thunder of chtinous clicks not a bang nor a whimper. Unless of course the PCs have rallied the forces of good (and possibly evil) and are ready to deal with it when it all hits the fan.
Or just run something premade. But heed the advice: don't plan too much too soon, players always derail big campaign plans. If you set this up and the PCs don't take the hooks then maybe someone else deals with it while the PCs get deeper into a struggle with the thieves guild and a family of fiendish sorcerers who want to take over the kingdom with fake coins that have spells/prayers attached to them.