Mordenkainen’s Fantastic Adventure (WG5), Greyhawk adventure for AD&D 1st ed, by Robert J. Kuntz and Gary Gygax.
“Four great and powerful heroes travel in search of adventure. Led by Mordenkainen, wizard without peer, they have come to mysterious Maure Castle, a forlorn and foreboding place, source of wonderment and awesome adventure, following a strange and terrible story.
For, somewhere deep beneath Maure Castle, from whence no person has returned alive, there stand two massive iron doors – doors without latch, lock, or handle – the Unopenable Doors.
And if the doors are opened, what then? Such potent evil, such terror lies beyond, that even the bravest adventurers may quail before the ultimate test!
Mordenkainen’s Fantastic Adventure is a challenging adventure for high-level play. Four original characters from Gary Gygax’ famous Greyhawk capmpaign are included for your use.
Can you meet the challenge? Can you survive what lies beyond…the Unopenable Doors?
An adventure for character levels 9-12.”