This item is actually an entire Palace! It is made of fine white marble and slender minarets. Since it is linked to the ethereal plane, it can only be called into the prime material plane via a magical key that fits the palace's front gate. The Palace can only be seen at night by the shine of its walls in near darkness. The chambers inside are well appointed with fountains and invisible servants who play music and attend to all menial tasks.
The original owner cursed the palace to ensure no one else could enjoy its splendor!
The curse takes the form of being hunted by an Invisible Stalker (see Monster Manual for stats).
This Palace can be used for so many different role-playing options. Options like finding the key and going on a quest to find out what it is for, or hearing about a Ghost Palace out in the deep desert and trying to find it! Imagine putting the players in a difficult position so that they must use or hide in the Palace to avoid some greater evil. There is something about hiding in the lap of luxury while worrying about some greater threat that just screams great role-playing!
Tips: As a DM, I would customize the beast/creature that stalks the owner of the key to ensure its something that would strike fear in even a hardy adventurer. An invisible stalker is suitable for even third to fourth level characters, but four fourth level players should be able to deal with one. Also higher level characters can escape from one with any dimensional spanning magics. I would also say that even if you slay the stalker that comes for the key-bearer, another will come after some suitable time. Nightly would not be out of the question, but that level of deadliness might be wrong for the tone of your campaign. Adjust as you see fit, of course, as with all things!
Finally, once it becomes known that the Palace has been discovered, many would come from far and wide for the chance to own such a treasure, so keep these kinds of things in mind when considering this for your campaign.