It's a new scenario that'll be released alongside the new WTC
People are saying it will be Umineko's episode 9 or something
IMO calling it Ep 9 is an exaggeration. Usually, when it comes to additional content in VNs, it's mostly a side or alternative story that is just a few hours long. The VN already pretty much concluded all the plot points, so I don't see a need for an actual 9th Episode.
PS: Also, should we maybe make a cosmology thread, like the one DC has? Tho I don't think calling it Umineko Cosmology would be correct, maybe not even WTC. Maybe something like "07th Expansion/Ryukishi07 cosmology thread"?