[quote=@Source]
<Snipped quote by Path>

It could be a trap, a farm, a mage with a grudge...
[/quote]

Who-or whatever created this has extensive knowledge of other beings who can travel between realms. This is a complex dumgeon that is similar to what I’ve seen. 
*looks at you*
I may have forgotten to mention this, but we were only able to see the demon door. The residents of this world only see boulders and foliage.