Realm of Chaos

From the back of the book.

Take up Excalibur, Jr., the magic sword, and prepare to enter the Realm of Chaos! A sorceror's curse has turned the kingdom of Avalon into a land of nightmare. Leprous fogs blot out the sun, putrid fungus attacks buildings, gold rusts, cloth rots and decay and death pollute the land.

The castle at Camelot holds the key to the curse, but the great gates are barred. You must discover the secret route that leads inside its grim walls. But to do so you must first enter the plague-ridden town of Glastonbury, where evil lurks behind every slime-encrusted door, as you seek the information and equipment you need on this, your most difficult and demanding Grailquest adventure yet.