The Poisoning of the Realm

Although the residents of the Lower Realms know the truth about the origins of the Upper Realm (a realm made by them collectively meant to be a paradise but proved too unpredictable for magic-wielding immortals), the mortals who populate that realm do not live such long lifespans and have forgotten their true origins. While most people can trace their lineages to such immortal beings, their understanding of why they left has been muddled by oral tradition.
Upper Realm Residents are only vaguely aware that there are other realms, but they are as distant and mysterious as magic itself. All anyone knows for certain is that the immortals once lived in their realm, but then left. There are countless variations of the story, but the most popular belief is that the realm existed as a utopia long before the immortals found it. The immortals poisoned the land with their magic. For a time, the mortals were seduced by their astounding deeds, even falling in love (or under the spell) with some of the immortals, and produced half-mortals with magic in their veins, thus polluting the mortal race.
As they watched their world decline, the mortals banded together, along with a few immortals who sympathized, and fought the magic-users out. It was a bloody battle and many died, but they eventually managed to drive the poisoners out. Centuries have passed since then, with the Upper Realmers trying to unlearn the dangerous habits they had adopted.
Though nearly everyone has heard this version of the Great Battle, and many believe it and in the need to abstain from magic, it is harder to act upon it than just believing. They are correct in believing that their realm is at the mercy of magic, but the amazing powers of a magically infused nature are often too difficult to face without some kind of counter-magic. It is the great paradox of their state, and many believe it is only a matter of time before magic completely destroys their world.