Several thousand miles under Silverfish, Great A'Tuin the world turtle sculled dreamily on through the starry night.

Reality is a curve.

That's not the problem. The problem is that there isn't as much as there should be. According to some of the more mystical texts in the stacks of the library of Unseen University - the Discworld's premier college of wizardry and big dinners, whose collection of books is so massive that it distorts Space and Time - at least nine-tenths of all the original reality ever created lies outside the multiverse, and since the multiverse by definition includes absolutely everything that is anything, this puts a bit of a strain on things.

Outside the boundaries of the universes lie the raw realities, the couldhave-beens, the might-bes, the neverweres, the wild ideas, all being created and uncreated chaotically like elements in fermenting supernovas.

Just occasionally where the walls of the worlds have worn a bit thin, they can leak in.
And reality leaks out.

The effect is like one of those deep-sea geysers of hot water, around which strange submarine creatures find enough warmth and food to make a brief, tiny oasis of existence in an environment where there shouldn't be any existence at all.

The idea of Holy Wood leaked innocently and joyfully into the Discworld.
And reality leaked out.

And was found. For there are Things outside, whose ability to sniff out tiny frail conglomerations of reality made the thing with the sharks and the trace of blood seem very boring indeed.

They began to gather.