The wilderness is an unextremely unexplored piece of land, north of varrock, but needs days of travel to reach. The Wilderness is extremely large and it houses Gielnor's mystical creatures such as demons, werewolves, dragons, revenants, etc, but despite this their numbers still are low and their is always the threat of each other.
There is several U.E.M.S.L.D bases or outposts that prevent humans from entering the wilderness or creatures from the wilderness to enter into the "real world". Any creature that bypasses the border, they are quickly incapcitated and thrown back into the wilderness.
Expeditions to the wilderness are banned due to past expeditions, which members don't come back alive, or few of them come back, and when they do, they are usually succumbed to insanity depression, or other unlikeable behaviors.