3) From Cheriza's description of it, it seems to be a different universe entirely. One that Kyouki (the being possessing Cheriza) wishes to take over. However, considering that Cheriza is insane, it could just be a fantasy world/hallucination she had, a product of her delusions (she and Sycor share the delusion that they can absorb Serenity's Taint) or a throwaway reference.
It's also mentioned in the Taint books within LS, where it's stated to be where the Taint originates from. There are a few old forum posts about it but the details are all really vague. In any case, I think there's good evidence that it's a real thing in-universe, but generally we don't know much of anything about it :3