Wishbringer
(available on the Masterpieces of Infocom collection)

In Wishbringer, you're a postal clerk in a small seaside village called Festeron. You deliver a strange envelope to a magic shop, and discover that an old woman's black cat has been kidnapped by "the Evil One." The old woman asks for a help, and when you leave the magic shop you find yourself trapped in a nightmare world. Your once-quiet town is now full of goons, trolls, vultures, fortress-like towers, and assorted wickedness. You become entangled in the struggle between Good and Evil; extraordinary help is found only in unusual places. Everyone seeks to possess a magic stone of dreams known as Wishbringer; but only you can find it and use its powers to make your town safe again. And you only have a few hours!