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[Borodin]

The Web is our friend. I got this off a website devoted to the Xeen titles:

"Worlds of Xeen is essentially one game, but due to some arcane marketing or production deadline issue, it was originally released in two parts: Clouds of Xeen and Darkside of Xeen. There was also a short add-on released later called Swords of Xeen. Today, all three of these games come bundled together and are installed simultaneously, so you can access all the areas in the game just as soon as your party can physically reach them."

So Swords of Xeen appears to have been a legit add-on from New World Computing, the original developers and publishers. Never heard of it before, myself.