I finally completed the Thieves Guild in Oblivion and I'm curious about the Cult of the Ancestor Moth. Has that order been in the game since the beginning or was it introduced in Oblivion? What are they exactly? Priests? Scholars? If they are a type of Priest which one of the Nine do they serve? How did their Order originally become responsible for the caring/reading of the Elder Scrolls? Were they already in existence and just chosen to take care of the Scrolls, or were they created specifically in order to watch over the Scrolls?

They are an order of Monks (or whatever the gender-neutral term for monk/nun is) dedicated to Julianos. Given the heavy metaphysical nature of the Elder Scrolls, I think the term Priest might be appropriate. As for their history, not much is known.

Finally, I believe that there were oblique references back in daggerfall to the Cult of the Ancestor Moth, though they most definitely were there by Morrowind.

They were said to walk around cyrodiil city in clouds of mothsin the PG1