There is a key spawned in challenge 6. After picking up the key you can go back to the house and open the door (you can die and you'll keep the key) once in the house you can go out a back door which will lead you out of the intended map and to the secret maze which will award you with an achievement. If you go to the tree in the maze you can die to it and it'll give you a piece of text from Zardy. This is all I've found so far ill keep you informed!