Mysterious Black Portal?

Maybe I'm just blind and have never seen this before, but where does this portal lead to? I stumbled upon it today. D: Pressed up, nothing happened, no message or anything saying why I can't enter it. It's in the Forest of Evil 1.

that's for the faust quest, there will be an NPC in there that you need to talk to at certain times of the day in order to complete the quest.