The swamp was once the center of the realm of the Emerald Magi. Purportedly their reign was brought to an end when a successful uprising of pleasure slaves replaced the magiocracy with pornocracy (rule by prostitutes). This spelled doom for the Emerald Magi and today the swamp is littered with their ruined towers hidding the secrets of the magi. It is an initiation ritual Marsh elves living in the swamp to steal something from the ruins.
Magic is still strong around the ruins. Therefore clerics, magic-users and elves need only half as much sleep and half as much time to prepare spells while in the swamp.
Note: This incorporates an entry from the Random Weird Historical Events table I won for my entry into the Hack Vornheim contest. Entry 50 to be precise.