*OOC note: This is not all direct fact from the DM. The information herein is more or less Kyrax's speculations based off the information provided (notes on the stone, Behold the Red Moon, and information gathered from the Elven library.) So it's fully possible that he is wrong on some things. I did my best to make sure that it was also obviously ICly where he was speculating.*
The stone in question is a mystery to me. I really lack enough information on it to really determine anything. Bits and pieces of these notes sounds like pieces of other legends I have heard, but I cannot say how many of them are rooted in truths, let alone if they're connected at all.
Interestingly enough though, it was claimed to have been a piece of the moon, or a moon. The Red Moon is a reference I am quite familiar with.
In the interpreting of dreams, the Red Moon is often considered to be a sign of death. And I have to wonder. When I was sent to the fort, it was mentioned that in the nightmares the guards began to have shortly after arriving, a Red Moon was a frequent occurrence. It could be connected.
Many cults in the past have use the Red Moon in their symbol, in the past it was believed to simply be due to it's common interpretation as a forbearer of death. But this other information, this translation of.. what? gives some new insights on that. It seems to be a prophecy of rather epic proportions.
Linking it to the notes on the red stone, which I can only assume you either have in your possession or know where it is, I would say it's something of an undead apocalypse. Which makes some sense, most of the cults which have used the Red Moon as their symbol were necromantic cults. Unfortunately the elven records do not indicate much better details than this, they are quite proud of their eradication of them. I'll check my private library soon, but I doubt I'll have much, most of my books are ones readily available anywhere.
If your Cult is related to this stone, and the prophecy. You mentioned they worship the Old Gods, and that you saw the symbol of Usezel in one of their .. temples? (you weren't clear there), I would have to say that at least they have read this prophecy to mean that Usezel will rise in power, casting aside all the other gods, False Gods to them, and bring about an end to the world as we know it. Casting it in to eternal darkness. Likely they believe that by following him and helping him to achieve this goal, they will be rewarded when he does.
Unfortunately, I simply have more questions than answers based off this translation. If you have access to the original of it, I would love to see it.
I was also able to find references to elves seeking .. something. But they never stated what, and it seems to have never been found. I would have to hazard a guess that it would be either the stone, or one (or more) of the keys.