Mappa Moon-day
This somewhat sardonic name for a magical artifact that’s over eight hundred years old is the sort of thing that you get when you don’t pay attention to what your grad students are doing. One jokester calls the item the “Mappa Moon-Day” at an absolutely critical moment during a sorcerous transport, and now that’s how it is defined by the universe. People have to call it that, if they want to activate the item at all.
And they want to activate the item. The Mappa Moon-Day superficially resembles an old medieval map of the world, but if you look at it under a magnifying glass, it immediately resolves as being an absolutely accurate map of the world. It, in fact, allows you to scry any area on the surface of the earth, at about the block level (and it can’t see through walls or ceilings). The item responds to mental desires: the user of the map can, through an act of will, somehow find the general area that she’s looking for (for some reason, women have an easier time using the item).
The only complication? Well, aside from being insanely valuable, to the point where people will start wars over it? The major one is that the item can only be used when the sun and the moon are simultaneously in the sky. Which is probably why the practical joker called it the Mappa Moon-Day in the first place. Well, aside from the obvious pun.