The Forbdding is a parallel prison dimension. The spot where the demons continuously emerge - the Hoare Flats, near the Breakline mountains - as Allanon states, is simply a weak spot that they can make use of to make crossing between worlds, or breaking out, easier.

Allanon explains in Chapter Eight of Elfstones:

For our purposes here it is only important that you know that the evil ones were defeated. Their power was broken and they were driven back and finally trapped. Those who had defeated them used their powers to create a Forbidding, a wall of imprisonment behind which the evil was to be placed. Their prison was not of this world nor any world, but a black hole of emptiness and isolation where nothing but the evil would be permitted to exist.

Many of the references to the existing breach by the Dagda Mor refer to the demons crossing between worlds, so this appears to be consistent with what is portrayed in Jarka Ruus.

The Forbidding is a parallel prison dimension. The spot where the demons continuously emerge - the Hoare Flats, near the Breakline mountains - as Allanon states, is simply a weak spot that they can make use of to make crossing between worlds, or breaking out, easier.

Allanon explains in Chapter Eight of Elfstones:

For our purposes here it is only important that you know that the evil ones were defeated. Their power was broken and they were driven back and finally trapped. Those who had defeated them used their powers to create a Forbidding, a wall of imprisonment behind which the evil was to be placed. Their prison was not of this world nor any world, but a black hole of emptiness and isolation where nothing but the evil would be permitted to exist.

Many of the references to the existing breach by the Dagda Mor refer to the demons crossing between worlds, so this appears to be consistent with what is portrayed in Jarka Ruus.