Planesglint is a very peculiar mental disease in which an individual is suddenly aware of some very specific knowledge, and is usually drawn to pursue a particular course of action as a result. Afflicted persons tend to abandon their former lives in their pursuits, and generally become drastically altered by their obsessions.
A suspected case of planesglint is the madness of King Drioc, which made him abandon his throne and all claims to his heritage in favor of founding a town in the middle of a swamp.
Cases of planesglint have been popping up ever since around 600 (Iron Calendar).