Can someone enlighten me what is the significance of something being a Known Thing in ESO? I've seen servants of Hermaeus Mora use the expression several times, both in a lorebook and in the Necrom chapter, so it clearly has some significance over and above all the other stuff the Woodland Man knows.

I came across this quote about Elder Scrolls:
"A deeper meaning is meant, too, but not very many laymen bother with that. Until a prophecy is fulfilled, the true contents of an Elder Scoll are malleable, hazy, uncertain. Only by the Hero's action does it become True. The Hero is literally the scribe of the next Elder Scroll, the one in which the prophecy has been fulfilled into a fixed point, negating its precursor."

Could this be related - is a Known Thing something that has become completely immutable and unchangeable by past, present or future actions whereas other bits of knowledge are "hazier", more subject to interpretation or ambiguity amidst the tides of Fate?

That would make Known Things roughly equivalent to the "fixed points in time" in Doctor Who, I guess.