Eh, I think it was mentioned in Ep 36.
And honestly, with all of the various fictions in which a "neutral observer" type character also happens to be some sort of immortal warden, it isn't and shouldn't be the most surprising of revelations. But it is something that was alluded to throughout the entire series, even if it wasn't directly announced.
That said, it is a similar vein to the elves being psychic, not magic, which is only mentioned in the GM turns and is all over the community forum with no spoilers.
But c'est la vie. If people want to post spoilers in one place, and then complain about different spoilers from the same source in another, that's their life choice, not mine.
But if there is a background book like people want, they're probably going to get spoilered anyway.
And it's also been like what, a month now? What's the statue of limitations on spoilers again?