Eventually, however, Man began to resent these creatures, these people with strange and mystical abilities. Strife between them ensued, and built upon itself until finally there came war: The one thing the race of Man was very good at. The Burning Times came and the slaughter that ensued threatened the very balance between magic and science. It threatened to render Mythos utterly extinct, and anyone who resembled them.
Queen Sylvanus stepped forward at this point and joined forces with the Emperor of the Underworld (at the time). Between them they managed to create a barrier that cut off from the rest of the world, making it possible to cross over into a place originally occupied by the Unimagined. Those creatures who never existed past the barrier, in what was now 'Reality'. Thusly the Otherworld came into existence and quickly became a haven for anyone who did not fit into the rigid rules Man now enforced in Reality.
Since then, the barrier has remained intact based on the need to survive. However, the actual dire need for it to remain so has come under fire lately. There are some who believe now is the time to remove it, to reintroduce Reality to Surreality and bring back the mystical elements long gone from Man's collective consciousness. Most prominent among them is Emperor Sistri, who wants to conquer Reality and add it to his domain. But there are others who believe it is no longer necessary to hide from Man, or that their ultimate survival depends on finding new blood.
On the other hand, there are those who do not want the barrier to be removed. Those who want to enforce Reality's rules want the barrier to remain intact, and will do whatever they have to to ensure it. And there are residents of the Otherworld who fear war breaking out and genocide on their own kind again.
(Again, pending approval.)