So many classics: A big freaking wolf eats the sun and everything freezes solid; The war between the forces of Heaven and Hell spills over to the material plane; Angels decide they had enough of mankind (and other humanoids) and march out against the mortals.
Let me think. I once had the players (being unwitting pawns to the Night Goddess Shar, of course) unwittingly opening a rip to the plane of shadow, which grow stronger steadily. If they handn't stopped the rip, it would have swallowed the whole plane and sucked it right into the plane of shadows, turning every living thing into shadows.
You could also crash an asteroid or a meteroid (or both) into the thing. A big one. The planet breaks apart, everyone dies.
Or let the fabric of magic unravel, the focus that binds it to the material world destroy.
"And if the resulting kablooie takes place atop the mountian, which is the hub of the world's magic field, it will, as I understand it, result in the field collapsing for... remind me, Mister Stibbons?"
"About two years," he said.
"Really? Well, we can do without magic for a couple of years, can't we?" said Mr Slant, managing to suggest that this would be a jolly good thing, too.
"With respect," said Ponder, without respect, "we cannot. The seas will run dry. The sun will burn out and crush. The elephants and the turtle may cease to exist altogether."
"That'll happen in just two years?"
"Oh, no. That'll happen within a few minutes, sir. You see, magic isn't just coloured lights and balls. Magic holds the world together."
Or the core of the planet will explode? Or, if that's too Sci-Fi for you, let the giant beast (the Tarrasque's Mother) wake up and eat its way out of its prison. (Did I mention that the Tarrasque's just a little baby practically fresh out of its egg and tiny compared to a grown-up specimen?)
