I’ve thought for many years about this and still can’t fully articulate it. So I thought that I’d try to start here...
When a Gloranthan uses their god’s magic they become their god. Mostly, we see this as the person participating in a fraction of their god. I don’t. I see them as actually becoming their god, with all the consequences that implies for the god.
There are a lot of issues with what I’ve just stated. And eventually, I’ll get to (some of) them.
First, I see this extending right back to the beginning of creation. The initial forces are very abstract and completely impersonal. After those initial interactions, the Runes arise - almost personality free and intellect free. Then the primal Gods, with a little intellect, choice and personality. Then the Gods that are worshiped, with personalities, choices, intellect and more human stories.
What I think is is happening is that the powers, the bundles of powers that we call the gods, and the personalities are entirely separate things, though deeply linked. I think that beings with personalities find ways to invoke those Runes and, by invoking them, embody them. However, it’s not permanent. This is hinted in the mythology of Orlanth, where at different times he has different names. My premise is that each one of those, and many more, is a separate being, a separate personality, that found a way to invoke the powers of Orlanth, and extend them slightly. Orlanth became them and they became Orlanth.
Vinga, a female personality, took up the the bundle of powers we call Orlanth and exercised them better than any of her brothers ( cousins, clansmen, tribesmen, nation, whatever ) and became Orlanth. Or Orlanth became Vinga.
This continues after Time.
The most famous case to my memory is Alakoring, who quite fundamentally changed the nature of Orlanth by making him more of a ruler and less of a first among equals. Less famous but as important and at the same time, Renvald Meldeksbane also changed the nature of Orlanth (but apparently less permanently).
How does it work? The key is identification. Some people are better at identifying with their god. It doesn’t matter whether this is because of ancestry, nature, luck, power, necessity, determination, a combination of all of these, or whatever.
So what does this theory do for us?
Well to me, it starts to explain how the God Learners were able to achieve the damage they did. They would search out devotees and priests of a god and find someone that was powerful and that they could manipulate. This manipulation would not be coercion or corruption, more likely naivety or greed. Coercion or corruption would lessen the identification of their victim with their god.
Having found their choice, the God Learners would offer a contest (a heroquest challenge) that they could genuinely lose. And then ‘cheat’ based on the personality or circumstances of their victim.
So, for the Goddess Swap, they offered a contest to two different senior priestesses, one in each land (likely, it was many more than just two, because the GLs could, and probably did, lose some). During the contest, the GLs would help their opponent identify more completely with their goddess. And then switch the basis of the contest in a way that their opponent could not resist. Perhaps a personality flaw that was consistent with the identification with the goddess. Or perhaps by posing a dilemma that was completely outside the domain of both the goddess and the priestess.
One part of their cheat was likely their RuneQuest Sight that allowed them to precisely identify the runes the opponent could manifest and which Runes they might be more vulnerable to.
Maybe the GLs even had magic that helped their victims identify more deeply with their gods and goddesses. And few devotees or priests could resist that...
Having won the contest, the GLs could the offer back the lost powers, on the condition that the priestess would relocate to a foreign land. As the priestess was her goddess, the two goddesses would have switched lands.
Similarly, this is how Tatius and the Lunars killed Orlanth.
I can pick some holes in my theory. First, some people seem to be genuinely powerful without that level of identification. Yes, and that’s OK to be independently powerful. Second, this doesn’t explain spirit worshipers that well (but with the right twist it might), and doesn’t explain sorcery at all.
In the meantime, I’ve already written a lot and power is rapidly draining from my iPad - maybe it’s the Doom Guardians coming for another God Learner...
Becoming the god - well yes and...
If defeated (or if unusually successful) then the level of identification matters. Not just personal identification but the communal belief or communal investment (power, treasure, etc) that the identification represents some wider community. Tatius 'killed' Orlanth in a very wide area because he could arrange ritual defeats of many Orlanth priests across a vast area, even if only symbolically by banishing worship from all towns and cities. This could impact Esrolia and Prax because they looked to Sartar and/or Heortland for Orlanthi leadership, even if only unconsciously.
At the start I mentioned powers and bundles of powers and then didn't much explain. Initially, the first personality to take up a power might express one Rune. Over time both the original personality and successors refine the expression of those powers and, particularly if successful or widely emulated, they expand the powers into bundles.
What gods know - gods know a little of what each of their communal worshipers know (which for a great god can add up to a lot), more of what their initiates know etc. They know this from when their worshipers use their magic. Basically, a Spirit of Retribution is self-imposed. This is also my explanation how the regional and local variations of the gods arise - a god does not always have global knowledge, so what their local or regional worshipers do shapes them.
Behind all the accumulated beings that make up the god, there is still a primal something with it's own agenda. This something, let's call it the core god, can be roused to give dreams and visions to those who will become heroes - or dead or insane.
It works both ways. As a worshiper uses more of their gods' magic, they take on more and more of the agglomerated aspects of their deity. If they resist this, then they lessen their identification.
Until a worshiper becomes illuminated... and then an illuminate can hide knowledge from their god.