This year in One Piece, it was revealed that Luffy's Devil Fruit was never the Paramecia-type Gomu Gomu no Mi, but rather the Zoan-type Hito Hito no Mi, Model: Nika. While this was already a huge twist, it was made even bigger with the knowledge that this specific Zoan Devil Fruit was of the Mythical variety.
When it comes to Devil Fruits, none are rarer than the Mythical Zoans, all of which grant their eaters the powers of some mythical creature or being. They are among the strongest Devil Fruits in the franchise, with many of the series' strongest characters having eaten them, most notably Kaido and Marco, as well as the aforementioned Monkey D. Luffy. Since their introduction, they have become far more important to the plot, especially during Wano, but unfortunately, they are still as mysterious as they are hard to acquire. One theory, however, might offer an answer to that mystery.
What Are Mythical Zoan Devil Fruits?
Mythical Zoan Devil Fruits are a unique sub-branch of the standard Zoan type. Zoan Devil Fruits are the most common in One Piece, granting those that eat them the ability to transform into whatever animal or creature corresponds to the specific fruit they have eaten, as well as into a form that's a hybrid of whatever species they are and that creature. With enough practice and understanding, Zoan users can even unlock or create new forms aside from the standard two they usually get, as Tony Tony Chopper has shown since his introduction. It has also been stated by the Gorosei that Zoan Devil Fruits, unlike either Logia or Paramecia types, have a will of their own that's related to whatever beast or being's abilities the fruit grants, making even the standard models special.
Zoan Devil Fruits in particular are stated to be perfect for fighters and martial artists, as they always enhance the eater's physical abilities in some manner. This is why most of the physically strongest characters in One Piece, such as Kaido and Rob Lucci, have Zoan powers. They were already strong without them, but their natural abilities have been enhanced further. While most Zoan Devil Fruits grant those that eat them the powers of "normal" creatures, like elephants and leopards, they also have rarer, more powerful sub-branches. The most common of these would be the Ancient Zoans which, while much more elusive than the standard models, are significantly stronger and grant eaters the powers of prehistoric creatures, such as dinosaurs and mammoths. The vast majority of those who have eaten this type of Devil Fruit belong to the Beast Pirates, led by none other than Kaido.
Speaking of the Yonko, Kaido's Devil Fruit is also a Zoan, specifically one of the Mythical branch. Mythical Zoan Devil Fruits are the rarest in all of One Piece, even more so than Logias, and grant those who eat them powers far beyond just transforming into an animal. Due to supernatural creatures and beings almost always having some manner of special powers, those too are granted to Mythical Zoan eaters. Marco the Phoenix, for example, doesn't just turn into a bird, but one made of pure blue fire. While the flames from this form don't burn people, they have healing properties, much like that of the Phoenix his powers are based on. In terms of power and versatility, Mythical Zoans might be stronger than any other type of Devil Fruit, even the Logias.
What Is God Valley?
God Valley is a place in the One Piece world that has seemingly been erased from history. As the site of an event most commonly referred to as the "God Valley Incident," very little other than that is known to either fans of the series or the characters in the world. Since that event, most of the island's history has been erased and has even been removed from all maps. Based on conversations between Vice Admirals, 38 years before the events of the manga/anime, the World Nobles and their slaves were on that island for reasons unknown. While there, the Celestial Dragons were apparently attacked by the Rocks Pirates, a crew made up of bona fide legends. A battle ensued that required Monkey D. Garp and Gol D. Roger to team up in order to bring down the crew's captain, Rocks D. Xebec.
Aside from that, almost nothing is known about the island, even though that conflict is what lead to Garp gaining the title of "Hero of the Marines." It is said by some characters that due to Xebec getting so close to achieving his dream of global domination in that battle, in an effort to avoid any sign of weakness, the World Government went out of its way to erase or cover up any information about that incident that wasn't in their favor, even the very incident itself. Due to this, it is still a mystery why the Celestial Dragons were there and if the Rocks Pirates had any other reason for being there aside from attacking them.
How Are Mythical Zoans Related to God Valley?
As strange as it might seem, a new theory has been put forth that might explain a deeper connection and motivation for all those characters being there at the same time. The theory posits that God Valley is the home of Mythical Zoans and that the main reason Xebec was there was to find Luffy's Devil Fruit, the Hito Hito no Mi, Model: Nika. While this might seem outlandish at first, there are pieces of evidence to support this theory.
Probably the biggest thing pointing to this being true is that most of the Mythical Zoan users have some form of connection to the God Valley Incident, even if only indirectly. Kaido was a member of the Rocks Pirates, and according to a flashback, he was given his Devil Fruit by Big Mom during the conflict. While Marco wasn't there himself, his late captain Whitebeard was, also as a member of the Rocks Pirates. Given how Marco was shown to be pretty young and have his powers by the time Oden joined his crew, it wouldn't be impossible to imagine that Whitebeard obtained his fruit from the island. This could also explain how the island got the name "God Valley," as it would be the origin of all Mythical Zoans, including those with the powers of actual gods.
The other aspect of this theory that could explain some parts of the event is that according to the Gorosei, the Hito Hito no Mi, Model: Nika is something of an arch-nemesis to the World Government and Celestial Dragons. With one of its past users having recently been confirmed to be Joyboy, it's only fitting that those who eat that Devil Fruit tend to become their enemies. Considering Xebec's ultimate goal was to overthrow and eradicate the Celestial Dragons to take over the world, as they themselves are implied to have done 800 years prior, his hunting for the Nika Devil Fruit makes a lot of sense.
Unfortunately, considering how little is known about the island or the incident, it's impossible to know at the moment just how true this theory is. The revelation about Luffy's Devil Fruit and even the God Valley Incident itself only happened during the "Wano" arc, giving very little time for the story to provide more context, though when it has, another new layer of information is always added, such as the reveal of Kaido's Devil Fruit. Only time will tell if this theory will be proven right, but given how One Piece is supposedly reaching its conclusion in the relatively near future, the wait won't be that much longer -- probably.