more on the light thing.Maybe it's not as magical of an explanation as i originally thought? mushrooms grow in dark environments because thwyre more likely to be able to sustain the level of moisture that they need. Sunlit areas dry out quickly which isn't ideal. and i don't think the great sage is going around watering these guys every day. so maybe the magical sunlight produced by daylight and weaker spells like the light cantrip speed up that process??? but then what about the sunlight up in the Feywild.
you know something interesting? when we stumbled into the Feywild in our very first session we did not see a single mushroom. there was the weird bleached grass and the pale sky and the forest that looked like all the colour had been leached out of it. No mushrooms though.
we are at the stage now that i think it would be beneficial to create a timeline of the infection. Because i don't think that the mushrooms and their spores were the main sources of disease anymore, not after a certain point.
the great sage used the rats (who were unaware of what they were doing?) to spread the spores across the channels of magic from the underdark to the feydark.
those spores infected whatever fey dwelled in the feydark and also grew into mushrooms which began to siphon magic from the wells of primal essence.
now, this is where it gets a bit more unclear. the infection would have had to spread to the main part of the feywild. And i actually don't think that it was the rats. at this point, the fey up in the fey cities would have heard whisperings of a new plague of sorts going around down in the feydark. perhaps the fey government would have sent a few officials to check it out?
and when those guys went down there they would have gotten infected too. As they returned to the surface in their corrupted forms, they would have been spreading the disease to every other fey crossing their path
(note? It's possible that the disease has increased in efficacy over the years. so initially, it was a lot slower in taking root, allowing the fey to retain some mind and direct themselves back to the surface. if not, it could also have happened by any mindless fey accidentally stumbling across an entrance)
the more undeveloped areas of the feywild would have been hit first, purely because of. Lack of awareness or knowledge of the plague?
this was all in conjunction with the great separation. So happening around the same time. And strife was already prevalent And tensions would be running high
an important point to speculate on here is when exactly the fey king was infected. We do know that the fey king and the great sage were in correspondence before the great separation. (side note. The great rat the great separation the great sage. dm maybe you should come up with a different descriptor for once/j)
perhaps there was a trap set for the fey king? He was lured to the material plane and infected there?
My theory on the fey king is for another day. i have many thoughts
now at some point those in the fey city will have realised that there is no point in going out and trying to save the infected. All they can do now is they and fortify the city to prevent themselves and the remaining fey from it