Plothook/Encounter/Campaign (it kind of does all three to varying degrees; I'm going into some detail here; this is the plot of my current campaign)
An evil, magically potent race across the sees in a continent only rumored to exist prepares to launch an invasion (in my campaign they're elves with a 16th century vietnamese vibe) to the players content.
The invading race stirs up rebellion in their chosen landing area and staging areas. They do this via doppelganger spies, and begin teleporting in monsters to terrorize the countryside and soften up the resistance. Then, with no warning, they use a souped up Locate City Nuke (the wight creating method) to demolish a huge area (strategically chosen), resulting a blasted hellscape inhabited only by wights and a lucky few survivors. This plunges, at the very least, one nation into utter chaos and economic ruin, and local leaders begin capitulating to the invaders. They use teleportation circles to transport their army across the sea (in my campaign, greater teleportation has a reduced range, so the elves use strategically positioned ships with teleportation circles to form a chain in order to get their army across the ocean.). The PC's then are faced with a choice - resist the obviously evil invaders, or join them, or just try to escape. My PC's were within the deadzone when it went off, and survived the blast only to be caught in a wasteland filled with hostile undead. They managed to escape, but were hired by a knightly order to scout out the wasteland to prepare for a strike force to rescue survivors.
This makes for a Fallout type campaign, and what you can do is basically restricted to your imagination. My PC's just barely survived a hellfire wyrm leveling the ruined city they were in. Even just the locate city bomb with the wight reanimation makes for a much different experience for the PCs, let alone the concurrent invasion and political strife.
EDIT: This also gives players the option to leave and do something else, so if they don't like it just adjust and do something else. Or have it happen far away, and have an NPC pay them to retrieve something from the deadzone. Or whatever, use your imagination.