I have just been horrified to read that there is a belief, common in the Magic: the Gathering world, that Nevinyrral's Disk was named for Larry Niven's Ringworld. This is not the case. In the short story "What Good is a Glass Dagger?" (currently available in his anthology Playgrounds of the Mind, though I first read it in one of the books in the Isaac Asimov's Magical Worlds of Fantasy series, entitled Magicians), Niven dealt with a fantasy world based on mana, which was a non-renewable (sort of) resource attached to locations. Anything magical slowly used up this mana, until magic could no longer be done there. Because wars at the time were fought with magical assistance (to the great detriment of the common foot-soldier), a pacifist group from Atlantis sent a thief to steal an item from the Warlock, and immensely powerful and clever magician who had created a disk. On it he placed two enchantments: one simply held it together, the other caused the speed at which it spun to increase geometrically. Thus, once it started spinning, it would not stop until all of the mana in the area was gone, and, because the rate at which it sucked mana increased so quickly, this would happen very soon after it was started. At the end, it would self-destruct from the incredible stresses of spinning so fast, and the failure of the enchantment which preserved it.

Interestingly, on catching this thief, the Warlock explains to him that Atlantis itself does not sink into the sea only because of the enchantments holding it up--if ever the secret of the disk were to get out, not only would war not stop, Atlantis would be exceedingly vulernerable. In order to make this point very personal, the Warlock took a glass dagger, and embedded it into the chest of the thief, with two enchantments on it--one kept it from killing him, or causing any pain, the other made it invisible and untouchable. Thus, the thief could not remove the dagger, but if he were ever to wander into an area with no mana (or create such an area), the dagger would reappear and kill him instantly.

The story is really quite excellent--I highly recommend it. It goes on from there, too--this is just the setup, so don't worry--I haven't ruined anything beyond the first few pages for you.