In the original lore, the demiplane of dread (Barovia's actual location) touches the prime material plane in places where great evil has existed for extended periods of time. It is a place created to sequester evils so powerful they could not be defeated by means available at the time so some entity (and on this my recollection of the lore fails) created a demiplane to trap these beings away.
In the original setup you could enter willingly, be tricked into entering or enter by mistake. But leaving, ah, now that was a problem since the whole place was a trap for powerful evil entities. There were also a number of environmental effects (limited access to divine spells, limited spell/psionic recovery due to the unsettling nature of the demiplane and a few others) that they skipped in the DDO implementation.