Heward's Mystical Organ is an enormous pipe organ as tall as a cathedral. It has three keyboards, twenty-seven ivory stops, and nine pedals. Each of its 77  gold and silver pipes binds an air elemental to play the appropriate note.
The twenty-seven ivory stops are arranged in three banks of nine, although some incomplete texts only mention thirteen of the stops. Each stop has eight possible positions.
The nine pedals are crafted from rosewood, and play deep bass notes.
Heward's Mystical Organ is always found in one grand cathedral or another, but never remains there for too long before moving on to a new home.
The abilities of the Organ as as countless as the possible combination of songs that can be played upon it. At least 23 ancient songs with unique effects are known to exist: five which empower the pianist, six which protect them, five enchantments, three which conjure some ancient being or thing, and three which bring down cataclysm. The last song is unknown.
One exceptionally brave team of researchers managed to safely test the effect of 128 possible combinations of seven stops. Effects included conjuring demons, casting arcane or druidic spells, teleporting the pianist to another realm, the Organ playing an automatic melody (at least 23 such melodies are known), conjuring gemstones, and absorbing all magic within the cathedral to recharge its magic.
Playing the wrong notes or making a mistake can inflicts an unpredictable backlash. They are stricken with madness, unable to bear the slightest of sounds. Setting the final stop to the wrong setting will also trigger the backlash.
Playing the instrument too perfectly is also dangerous. Anyone who hears the hauntingly beautiful notes of the Organ risks being enchanted by them forever. All other sounds, all other things seem flat and meaningless. Eventually, they no longer care for food or drink, and gradually waste away.
The Organ's location in the world is constantly changing. Anyone who leaves the Organ's hall is certain never able to find it again.
As an artifact, Heward's Mystical Organ cannot be destroyed by normal means.
Rumoured ways to destroy the Organ include convincing the King of Mice to lead an attack on its inner workings, burning it in a fire kindled with a copy of every work Heward ever composed, and having the worst composer in the world play the Organ, causing an explosion which levels everything within half a mile.
According to the Fables of Burdock, the god Heward created the Organ to bring harmony to the world through music. But mice gnawed the machine and brought discordant notes and dark magic. Heward forever cursed mice to be quiet of voice, and abandoned the Organ.
Heward was a mortal bard (as he would be known today) who ascended to the humblest ranks of divinity in an ancient time when the Age of Elves was fading. Heward watched that race decline as mankind rose to prominence over the coming centuries, and feared that the beautiful elven art of music would soon fade from the world.
Over a period of years, Heward created his Mystical Organ, an incredible musical instrument whose sound sampled the voices of nightingales and inspired all who listened. From this, mankind learned the secrets of musical composition, and the first bards arose.
But the gods, the elven pantheon among them, were jealous of Heward's popularity, and resented him for giving the secrets of composition to humans, who they feared would abandon the ancient elven music. They commanded mice to sabotage the magical instrument, turning its notes dark and making the instrument too dangerous to use.
Many bards boast that they have played the Organ, but most who truly encounter the artifact in some ancient cathedral are not reckless enough to attempt even a single note.
Oldenburg the Blind learned the enchanting to win the heart of Princess Leir, daughter of the evil faerie lord Marrad. Not all users are so lucky. Mad Ossam was cursed by the Organ, and blight and ruin followed him wherever he went.
Heward's Mystical Organ appeared in Eldritch Wizardry (1976). A later expansion describing its powers apeared in A Working Design for Heward's Mystical Organ (or, "Play it Again, Sam"), Dragon Magazine #29 (Sep 1979), p.43.