To access this demon door you'll need to be fat and ugly. Here is what to do:

1. Wear a set of pajamas, then color each piece a different color.
2. Eat meat & pies which can be found at the local shops. (you can also drink beer)

Once your good and fat (check your status in the Sanctuary) simply show up at his door with the new you and he will give you access to a treasure chest which holds 40 Guild.