The Urn, sometimes referred to as the Therian Urn or the horcrux pot, is a magical cursed artifact which, as long as it survives, preserves the soul of the wizard who created it, much the same as the One Ring (Tolkien's Lord of the Rings). It is called "therian" due to the nature of its curse, exchanging one kind of transformation for another.
The story goes that it was created by a wizard thousands of years ago as a means of increasing his own power. It has now been revealed that the Urn is the horcrux of the evil wizard Malaku. During the evacuation of Egston's kingdom, Malaku placed a portion of his soul into the urn, then hid it on Xymel Earth, as seen in book three of Room For One More. By some unknown means, it made its way to a garage sale as a cheap object d'art and was given to Bob by his father as a gift. Woofy introduced the Urn as the primary plot device in the webcomic Quest of the Therian Urn, but it may have a connection to Otter Island as well. The wizard Nikito, after learning that Malaku had created a horcrux, became the Urn's worst enemy and sought to destroy it, but his efforts were thwarted by the fact that the Oberon's Gate on Xymel Earth had been moved to another planet. Instead, he uses his precognistic abilities to guide allies in the future in hopes that they may succeed where he has failed.
The urn is a small round pot with a short neck and a flared opening, just about the right size to hold the cremated ashes of an average human corpse. Its color varies according to whether its power is at its peak or ebb. When its power is strong, it is either black or some other dark color with a white streak running down one side. When its power is weak, it turns white with figures depicting humans, beasts, humans being transformed, and other symbols.
The urn's primary power is to take ordinary or useless matter and transform it into something attractive. It seems to sense what the user wants and caters to his greed. Once the user's "wish" is granted, the urn begins to extract its price in the form of the curse. The joke is on whoever was foolish enough to continue putting things into the pot, who quickly begins turning into some kind of beast. The transformation is not complete as it appears to leave the mind intact as well as the capacity for human speech and, in the cases of Bob and Lucy, their thumbs remain functional. Bob's explanation is that they were not as greedy as their other friends.
Like the One Ring, the Urn is magically protected from damage by mundane attacks such as being hit with a hammer or dropped from a rooftop.
There does appear to be a secondary power connected to the urn--that of subliminal influence--as Bob's father remarked it was "begging" to be given to Bob. Taking the magical nature of the object into account, this was almost certainly more than coincidental.
Malaku's particular brand of magic relied heavily on moonlight. Because of this, the Urn loses its power for several hours during each new moon, temporarily returning humanity to anyone who did not give theirs up willingly.
Because it is a horcrux, the ultimate power of the Urn is to return Malaku to life once it has absorbed enough humanity.