The Elbaph arc of One Piece has revealed quite a lot of information about the history of the world, including the Void Century and some powerful Devil Fruit users from it. Thanks to the Harley, Oda has dove deep into history, but more than that, the lore of this country also sheds some light on entities that existed long ago in the past, such as the Nidhoggr. Thanks to the existence of Nidhoggr, as well as Imu's deep connection to the beast, it has now been established that the Void Century saw this dragon soar in the skies hundreds of years ago.
One Piece Chapter 1175 established a deep connection between Imu and the Nidhoggr dragon. When Loki emerged from the Underworld of Elbaph, he started wreaking havoc in the Sun World almost immediately. Loki was able to crush Saint Sommers and, at the same time, destroy the vast majority of the MMA that were running amok in Elbaph. However, Imu was clearly not fazed by Loki's abilities. Imu even faced up to the Nidhoggr, stating that it was about time they found out where the Nidhoggr was.
The way Imu spoke about the Nidhoggr felt as if they had a deep history. In fact, it can be said without a shadow of a doubt that Imu and Nidhoggr have some history. Ever since the conclusion of the war, the sovereign of the world has been searching for this ability, which means that the Nidhoggr was active at some point during the Void Century.
"To think that I would see you again in this world, Nidhoggr, you traitor. And snow, ah, indeed. Lightning, caused by impact with ice. Your affinity for one another was always great, Ragnir!" - Nerona Imu highlights the synergy between Loki and Ragnir, One Piece Chapter #1182
Imu having a history with the Nidhoggr means that this ability must have been active in the Void Century. If the God of War of Elbaph died thousands of years ago, then Imu would not have known the Nidhoggr at all. This is simply down to the fact that immediately after the Nidhoggr died, the Ratatoskr’s soul resided in Ragnir and started defending the legendary Devil Fruit of the country ever since.
Loki is only the second user of the Nidhoggr Devil Fruit, meaning that the God of War must have been alive up until the Void Century. As such, Imu and the former God of War of Elbaph must have had some history and could even have clashed long ago.
After the death of the God of War, the Devil Fruit wielding the powers of the Nidhoggr was hidden by the Elbaph Giants. As to why that is the case, the reason is rather simple: allowing this power to fall into the hands of the World Government would bring forth catastrophic consequences. In fact, Imu already tried to do that once with King Harald, and again, later on, with Loki. Clearly, Imu has been after this Devil Fruit just as much as he's been after the power of the Hito Hito no Mi, Model: Nika.
And now, Ragnir, after an eternity of standing guard over the fruit, has finally chosen Loki! — Jarul, One Piece 1175
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