So my fantasy world (name WIP), connects of the human world. It's mirrors our world, however none of the creatures in it can escape. The only way to get in is for a human who possesses some sort if magic to to open the barrier. Once you have gotten inside the world, you cannot leave without help of a Scorrwin. 

So, how did it get like this, you may ask? Well once apron a time bored worlds where connected. You could travel freely between worlds, and humans and magics lived together in peace. The one's who held the barrier open where the Scorrwin. It took their magic combined to hold it open, and without them there would be no connection. The magic world in question is a living being, and it strives to keep the magical creatures inside and the non magics out. The Scorrwin where the ones stopping it. However, illness swept through the Scorrwin population, Killin off all but one, a small vulture like Scorrwin spawn. Without the Scorrwin to keep it open, the barrier closed. All humans trapped inside where eventually killed off by the island, leaving only the magicals and non magical animals.

The one Scorrwin left only has enough magic to send creatures into the humans world, not keep the world open. However, most creatures are content with their home and don't want to leave. Once in the human world, the magic world will try it's best to lure the magicals back into it world, and it is very difficult to avoid it's traps. The humans have gone so long without this world that they think of it as a myth.