Ad Astra Per Arcana
The Spelljammer is a legendary ship which looks like a gigantic manta ray, and houses an entire city on its back. All spacefarers (people who live in wildspace) have heard of the Spelljammer but very few have ever seen it themselves. It is this ship that gives its name to “spelljamming”, “spelljamming helms” and anything else connected with spelljamming. The ship has been reported to have been seen in countless spheres for as long as records go back. Even some groundlings (people who live on planets that have very little or no commerce with spelljamming communities) have legends about it. There are hundreds of conflicting legends about this ship, and a mythology has developed about the ship that is similar to the legends surrounding The Flying Dutchman.
In certain rarified circles it is sometimes speculated that The Spelljammer is one of The Ancient Brethren like the The Serpent with whom Vecna had his pact.