I think we should be able to universally agree that the biggest long-term revelations from chapter 957 are all things related to the “God Valley Incident”. I guess this can be a discussion about this as a whole since I will cover various aspects of this, but per the title I want to focus on the key details
Before getting started, this is all based on the undeniable context that the moon residents are directly tied to the Ancient Kingdom and possess Vegapunk-level technology from potentially thousands of years ago.
On the Birka Illustration on the moon, we see three people, representing the three Winged Races. However, below the ‘Skypiean’, there is another person – thus indicating that there was a Fourth Moon Race – the D. People.
So with the preamble out of the way, let’s get started with some of the clarification stuff first. A common reaction was God Valley = Raftel. This cannot be true because way too many people have been there.
In the above panel, we have random common marines present. It is very likely that Tenryuubito, their slaves, the Rocks crew, the Roger Pirates, Garp, and the potential mass of common Marines all have been to God Valley.
Too many people know its exact location for it to be Raftel. It sounds like the only people that have been to Raftel are Ancient Kingdom residents, the Roger Pirates, and maybe some Tenryuubito/Government leaders (Gorosei, Im, etc.). Moreover, based on the map below, because it is in the middle of large islands, there is a high probability people randomly encounter the island travelling between the numerous islands that surround it.
On a side note, that big island on the left side is something completely new. The largest island shown on the official map (presumably Elbaf) is big enough to span the entire side of the Calm Belt. So this is an entirely new area of Oda is updating things. I would lean towards the former because Oda probably only gave us a partial map, and this new section is probably just a small part of the remainder.
The raw does a better job of explaining that the “disappearing” of God Valley. Sengoku’s words are that God Valley is an island that the World Government hid or made disappeared. This is the panel below the map on the image below. So Oda, in his typical fashion, is leaving all options open. My personal bias is that the Government is just hiding it. This is based on the meat of the theory below.
Like most if not all Government/Tenryuubito bases, they all have direct ties to the Ancient Kingdom. Mary Geoise used to be an Ancient Kingdom gathering ground of great importance. They made elaborate and large art on the staircase they carved out (the Red Line is indestructible by current technology). Since this is the highest point in the world, maybe there is some sort of nostalgic aspect on being the closest to their old residence on the moon.
God Valley appears to have similar cultural/religious importance to the Ancient Kingdom.
With the silhouette of the island, we see that the island is named after the massive valley that looks to span the entire island. My bias is that this is naturally occurring, and it was the beauty of the valley that brought the Ancient Kingdom residents to build a base there.
The next key detail is that the Tenryuubito, and potentially all of them, are gathered there.
This also sounds like a regularly occurring event because Rocks was able to plan a massive assassination plan because he knew they would be there. Because of how stuck-up they are, they do whatever possible to avoid each other unless it is something they really, really want to do. Thus, for all of the 19 Tenryuubito families to gather, something important has to be going on.
This is where the options are endless because of the level of technology the moon residents (and thus the Ancient Kingdom) had. My gut leans towards the beauty/art aspect. With such a large valley that spans the entire island, extreme sun light mechanics could be produced. I think the Ancient Kingdom created a work of art around mother nature (a common occurrence in the real world too). On some sort of yearly basis, when the sun is at the perfect angle to pass through the valley, the world’s most beautiful light show gets put on. Thus, the name God Valley. Only God could produce something so beautiful (of course through the assistance of whatever additions the Ancient Kingdom did).
Because the Tenryuubito see themselves as Gods, they want to keep this view only for themselves. Thus, the Government/marines are ordered to keep it hidden, and to prevent all potential travelers from getting to the island.
This is HUGE because if Blackbeard learns of Im and the real King of the World, I wouldn’t be surprised if he follows Rocks’ path. He will have the power (the Gura abilities) to assassinate all 19 Tenryuubito if he learns when the light show occurs.
So God Valley is the opposite of Raftel in many aspects, but carries a similar level of impact on the series. Not many things come remotely close to Raftel’s level of importance, so the title of “ultimate” has many levels/ties. “Literary perfection” is the only way I can describe it. Most people will only see the Yonko aspect of “ultimate”, but I’m really thinking “ultimate” refers to the “ultimate” assassination attempt that comes up in the future (in addition to the “ultimate” battle that occurred there).
*Theory by Seiryu