I believe I have officially fallen into the proverbial deep end, because I have recently read some parts of King of Sartar
So there are two events that come to mind:
1. Orlanth killed Yelm and caused the Storm Age and Lesser Darkness. But that resulted in serious problems for mortals, culminating in the Great Darkness and the need to bring Yelm back.
2. Argrath (or a bunch of heroquesters under his name) took down the Red Moon and also killed Wakboth, causing the 4th Age. The people from the 4th Age seem to describe a world constantly getting worse and lacking magic, which sounds analogous to the Storm Age and the Lesser Darkness to me.
If the Red Moon is the embodiment of the Moon Rune and Wakboth is the embodiment of the Chaos Rune, then would this not be removing two things that Glorantha really needs to function? Without Moon Magic there might no longer the regular maintenance of the Mortal Realm and its laws of reality by connecting it to the God Time. Without Chaos there might no longer be a source of energy and mass for Glorantha itself - since all originated from Chaos in the first place.
The 4th Age is probably not headed for a "Darkness" but it may at least be headed for some kind of other un-survivable condition, like a magical version of heat death of the universe, and lack of any new souls to inhabit newborn mortals, and inability of anyone to do anything heroic to fix anything.
Am I approaching Illumination about Glorantha's fate in the case that King of Sartar occurs as written, or am I way off-base here? Would the 4th-Agers then need to do a heroquest of the Moonchaosbringers to restore habitability again? Of course creating someone other than Wakboth to be the embodiment of the Chaos Rune might be nice.