Not a place, an artifact. The mark of rulership of the Empire, in possession of the Emperor. [Or Empress as the case may be; Aerich's native tongue lacked as many gender markers as English.] Every citizen has a link to it, though often not the education to use it.

It is how I know we are not in my world. If we were...

[ He shrugged; if they were, he'd be able to use the Orb. Not well, as he was not a sorcerer of note. But it would also suggest methods to find out where they were, and the state of the Empire, and certainly his disappearance might be noticed by someone capable of using the Orb to telepathically find and communicate with him.]