Mirror of Oversight

The Mirror of Oversight was a magical mirror located at Unseen University. It was mentioned in The Light Fantastic as a scrying device used by several senior wizards to attempt to locate Rincewind. One wizard tried just the Mirror, and another (in an attempt to out-fail the others) tried this Mirror, the Runes of M'haw, and the entrails of a manicreach to find Rincewind. Both failed, and no other wizard who used different methods succeeded. This was a symptom of the beginning of the failure of magic across the Disc, as Great A'Tuin neared the Red Star.

Given the name of the Mirror, one might suspect that it was originally built by a Wizard to keep an eye on lesser wizards in his order. Probably a good idea, given the traditional method of promotion for lower ranking wizards (that is, killing all the higher ranked wizards).