Spirit World.
As the name suggests, only spirits of the netherworld were able to set foot in there.
Of the Four Sides, only the Spirit World was difficult to go into.
When the Middle Realm fell into chaos, only this place remained peaceful, cut off from the rest of the realm in solidarity. Only the Dark Emperor had set foot inside the Spirit World and besides that, no other person alive had ever taken a single step within.
The Four Sides had never came out easily. Even the Nine Temples and the Twelve Palaces knew very little if anything of the Four Sides. In the Middle Realm, instead of saying that the Four Sides were four sets of powers they might very well be known as four legends. Few knew anything about them but lingered on in the minds of people, full of mystery and miraculous.
Everyone only knew that the Spirit World was one among the Four Sides but no one knew where the road leading to it was.
