I was reading about the Leviathan on Wikipedia a while back and stumbled upon some interesting details.
The overview reads:
The concept of a chaos serpent may seem familiar, because in the Sistana arc we learned that the dragon handle part of the Coffin of Eternal Darkness represents a Dragon of Chaos. This entity was apparently important enough to be depicted alongside the major clans, minus the demons of course, as one of the pieces of the Coffin:
The fact that this entity is connected to the dragon handle is an important detail, because we have previously seen a massive ghostly dragon briefly manifest from it, seemingly invisible to everyone present:
Originally this scene could have been interpreted as Meliodas exerting his "Dragon's Sin" aura to scare off Oslo. However, in light of this new information that the handle represents an actual creature that once existed in the world, it seems likely that there could be something more to it. The Coffin was crafted by Dubs, who in the 2nd movie used a staff that managed to seal Meliodas and Elizabeth within it. Perhaps the dragon could have ended up sealed within the handle in a similar manner. This could also mean that Percival using the dragon handle sword is not just a parallel between him and Meliodas, but actually an important plot point that relates to the Dragon of Chaos potentially breaking out of its prison.
It's worth paying attention to the dragon's appearance, as both the handle itself and the ghostly apparition that manifested from it appear to have no arms or legs, which is how sea serpents like the Leviathan are typically depicted. The fact that Nakaba chose a serpent-like design for the dragon is quite interesting given the Biblical elements in both NnT and the sequel. The Wikipedia article on 'Serpents in the Bible' reads:
Based on this, it's possible that the Dragon of Chaos was inspired by that famous serpent in the Bible. Funnily enough, that would mean that the early state of the world of NnT was actually somewhat similar to the Biblical Genesis. You have Chaos representing God and then you have a man(the Demon King), a woman(the Supreme Deity), the tree of knowledge of good and evil(the Sacred Tree) and a Serpent(the Dragon of Chaos). You could interpret the DK and the SD sealing Chaos away as a similar act to Adam and Even disobeying God, but personally I think this is too loose of a connection to be meaningful, at least for now.
All that stuff is cool enough on its own, but it gets better when we look at the section of the Wikipedia article that discusses the Leviathan as it relates to Christianity in particular:
This suggests that the Dragon of Chaos could actually be the dragon in the Book of Revelation. In case some people are not aware, the Book of Revelation describes the events of the Christian Apocalypse, which is where the Four Horsemen of the Apocalypse originate from. Given that both the manga itself as well as the main group in it are named after the Four Horsemen, it seems relatively likely that Nakaba would also include some other elements from the Book of Revelations in this series. This means that the Dragon of Chaos might actually be the devil, who appears during the Apocalypse and is defeated by God at the end of it. Normally the idea of Satan himself appearing in a story like this would sound silly, but we are dealing with a series that has already referenced many other Biblical concepts and that is specifically named after elements of the section of the Bible in which Satan is finally defeated. If the Dragon of Chaos really is based on Satan, then it's quite likely to be the final villain of the series given the whole prophecy about how the FKOTA are going to trigger the Apocalypse. That said, Satan is supposed to take on the form of a seven-headed dragon, but the dragon handle depicts a dragon with only a single head. Perhaps the Dragon of Chaos is merely the Leviathan after all, or maybe the dragon handle only represents/contains one of Satan's seven heads that were somehow separated from one another.
To wrap things up, I want to briefly bring up a couple of details that may or may not be related to all of this. Firstly, one of the databooks apparently stated that Zalpa, the tribal who saved Matrona, respects 'the dragon god'. I say apparently because I found this info on the Wiki and they did not include a source, so take this with a grain of salt. This is the only mention I could find of some sort of a dragon god, but it definitely sounds like it could be referring to the Dragon of Chaos. This would presumably mean that this dragon or at least a part of it is involved with the savage tribes. I wouldn't mind something like that to be honest, because the tribes always seemed like such a random element in the setting. Secondly, most readers probably remember the giant skeleton mountain that Sin teleported Percy and Donny to in chapter 5. Donny called it "the dragon's backbone" and it looks like the spine of a gigantic serpent. That seems like it could also be connected to the Dragon of Chaos, as the mountain was hundreds of times larger than the other dragons we've seen in the series. This a very loose idea, but perhaps this means that the Dragon's body(the skeleton), mind(the dragon god?) and soul(the ghost in the Dragon Handle) have somehow split apart and become separate entities? Or maybe each serpent is a different head/aspect of Satan taking on a different form? Who knows. Like I said, these details may or may not be related, but I thought I'd mention them anyway for the sake of completeness.
TL;DR: The Dragon of Chaos that the dragon handle represents might be the Leviathan or perhaps even Satan. Final villain material.