Gederða (concept)
Gederða are a strange metaphysical anomaly that strikes the world seemingly random at random times. Wide ranging magical waves of unknown origin, Gederða have done everything from altering human mental states to tearing down mountains to sinking entire continents depending on their severity. The most important one to modern history was The Odrstund which struck Aversaria in 1168 IS and greatly heightened the vice's of the realm's Magi. This matches a trend of most Gederða effecting Magi more than mundane humans.
While their cause is unknown, several scholars have noted they seem to be getting more severe and more frequent as history has continued, their rate growing rapidly since Frodbrokna.