From a past that stretched beyond memory, there existed in that place a pool. On the surface it was like any other pool, nestled amongst the rocks and trees, sunlight glinting off its surface on the days when the sun indeed shone through.
There was no reason to believe that it was any different than any thousands of others like it scattered across the earth. But it was different. Very different.
If one were to come to this pool, and sit down for awhile, and study it for a time, one would beginning to notice some odd things. One would begin to notice how no insects or birds would come to this pool. One would begin to notice how, if it rained, not a single rain-drop would fall upon the water. One would then notice that any leaf that fell in the autumn of this place would fall away from the water. Finally, were one to actually lean over, and peer into the depths of this pool, one would then begin to see not stones, or mud, or plants or fish. One would see a thick and oily morass bubbling just below the surface, in which, now and again, faces of nightmarish evil would peek.
So this pool had sat for millenium upon millenium, from before the time when human kind stood upon the earth. A mirrored trap for creatures of evil and demons from beyond. Safely ensconced in a magical field that kept all outside influences from breaking the surface of this prison.
Then, one beautiful, sunny day at the end of an Autumn at the end of an age, a single leaf broke off its branch, and descended towards the pool. As it fell, this time, it did not fall away. It did not somehow take a course that would lead it to the bank. This time, it fell straight down. This time, it would touch the surface of the water, and as it did, the surface rippled outward and shattered.
The magic was gone, and the end of the earth poured out.