Mike's Magical Monical

A fancy-looking monical that allows its wearer to gauge another creature's magical ability. When wearing the monical and having attuned to it, a creature can make an arcana roll against another creature to visually see its magical power. This comes at a cost, however, as using the monical too often or against an especially strong creature, particularly those who are versed in radiant magic, may cause blindness in the eye that looks through the monical. The monical will also detect the magic of equipment or weapons that have been attuned.
At the DMs discretion, a Constitution saving throw must be made after using the monical a number of times, with the DC starting off low and gradually becoming higher after subsequent uses. Depending on how unsuccessful the saving throw is, the user of the monical will be temporarily blinded in one eye for a duration, or even permanently if a nat 1 is rolled for a DC of 10 or more. The saving throw may also be called for when investigating a very high-level magic user. Partial blindness will result in the creature having disadvantage on perception checks, their passive perception cut in half, and disadvantage on ranged attacks.