Sandy Petersen c. 1994 said:
THE GOD LEARNERS' STICKY END
The God Learners, at first, were Heroquesters. They went everywhere on the heroplane, and learned all the ropes. They also developed a certain mindset -- the gods were expressed as collections of Runes, spells, and POW. In a sense, they approached the world of Glorantha just like a minimaxer. They _were_ minimaxers. Scientific ones, but minimaxers.
To learn more about the hero plane, they did many experiments. The Goddess Switch, Temple of All Trickster, and other such are well-recorded. Other, more successful, projects of theirs are still around today (Caladra & Aurelion, the suppression of Teleos, etc.) In the process of their learning, they eventually discovered a most powerful secret -- a "trick" if you will that they were able to use to do amazing magical feats. Few could withstand the God Learner magic in warfare. Certainly the Waertagi could not.
Now, as they continued their heroquesting and experimentation, they gradually began performing the equivalent of destroying the ozone layer -- as heroquest paths were destroyed forever, or merged with others, so the Godplane began to alter. The Godplane is a complex place, and the God Learners were unable to predict the exact results of their actions (just as Western civilization was unable to predict that one effect of widespread DDT use was the near-extinction of many bird populations).
In the end, the Godplane was rendered unstable. After a while, it "shifted" on its own, moving back to a new, stable position. The entire nature of the universe was changed. One of the changes was that the God Learner Secret was no longer possible to exist. Yet it still existed. The universe reacted by sending the Gift Carriers to expunge every single vestige of the now-impossible God Learner Secret. It was part of natural law, like gravity. That is one reason that the destruction was so complete -- the God Learners themselves acted as weak points in the universal fabric that attracted "antibodies", which proceeded to wipe them out.
THE MONOMYTH
One of the God Learner "secrets" was that the Monomyth was/is TRUE! They found that when they went on an Orlanthi heroquest in which he slew the sun, they could "jump tracks" at the Solar contest, and now go along the Yelm side of the quest. There's been a lot of spouting about how there are different aspects of reality in Glorantha, and the Sun God is "Elmal" for the Orlanthi and the "Sun Dragon" for the Kralori, etc. but the God Learner usage of creative heroquesting demonstrated the essential unity of Gloranthan myth and made sense out of the jungle of fairy tales and legendry that had existed up to that time.
Now that the God Learners are all gone, the Monomyth is gradually disassembling, and the once-useful unity is degrading back into the myth-jungle that existed before Arkat's time. The reassertion of local beliefs that occurred at the end of the Second Age (re: the destruction of the False Dragon Ring and the Six-legged Empire) also played its part in the Monomyth's break-up.