Mammon runs his thumb along the forged adornments that made the coin, to some the expression on the devils face would be described as revere others would describe it as greed; for this coin shines more then any other coin in Minauoros. 

Deep with in an ancient dwarven stronghold, where the forges have have long gone cold and stillness holds dominion, Oppressing all that lie within; There is a great treasure vault, its doors long smashed off their heavy adamantite hinges; Lies a mountains of gold... It is said sitting somewhere in this mountain is a coin that once belonged to a god. A coin that will grant a wish, though be warned that all who have tried to get to these halls to claim this coin have perished and where never to be seen again. 

A gilded coin in the that depicts the many sides of a dodecahedron, on one side is 20 on the other is a 1. To find one of these coins is extraordinarily rare. The origins of these coins are unknown.

You can use an action to cast the wish spell from it. The coin disappears after you do this (hand the coin to the DM).

Optional:
Heirloom (Meta-Item): Heirloom items should be seen as more of an optional rule that the DM and Players can toggle on or off for certain games. If toggled off, no heirloom items may be used. At the discretion of the DM, this item can be passed on between any character that the DM is currently running, this could mean that a character in Faerun could use it or the player could use it for a different character in a campaign in Eberron. This optional function would be suggested to give to a player as a gift from the DM. This would also mean that the player could use it whenever they liked.  This should be treated as an optional rule where players & DM can decide to toggle Heirloom items on or off.