I belive that Imu has THE devil fruit. It can be mythical zoan model Lucifer, Satan, whatever. The power would allow Imu to imbue demons (yokai, etc.) into humans (including themselves). Similar to Moria’s fruit but more powerful (i.e. Ace and Sakazuki).
The fruits powers go as follows:
● Imu (the wielder) summons a demon from the underworld and fuses them to a host.
● Since they are from the underworld, they are already dead. You can’t kill what’s already dead, which explains the perceived immortality and damage invulnerability of Imu and the elders.
● The person who is fused with a demon can use the powers of and transform into the demon.
● In addition, since their bodies are fused, they can be summoned from place to place by Imu (i.e. the elders arriving on Egghead using summoning circles).
● Since the demons are connected to the humans through Imu, they can communicate with each other telepathically with Imu acting as the “server” (which receives and distributes information).
● Finally, the demons can be separated from the host, removing the immortality of the host. In turn, killing the host as the natural flow of time catches up to them.
The in-lore origin: Knowing that the devil fruits originate from powerful wills, it can be assumed that the fruit’s powers originate from a person with an extremely malicious will to create “hell on earth”. This character could have been the enemy of the original Nika and it was a battle between light (heaven) and darkness (hell). (This is an extremely simple explanation, but can easily be further developed)
Further thoughts and ideas:
● I don’t believe that Imu’s fruit powers are tied to erasing or re-writing. The thing that we have seen Imu “erase” can be summed up to the phrase “history is written by the victors”. That power is inherited by being the ruler of the world. It’s control of information. Lulusia wasn’t erased, there is a giant hole in the sea where it used to be. The void century is a result of the great flood caused by Imu and the remnants can still be found on the bottom of the ocean. The poneglyphs are still around. And people who knew of Saturn won’t magically forget he existed.
● Imu intentionally named the fruits as “devil fruits” in order to hide the identity of their own fruit. During the void century there might have only existed one fruit known as THE devil fruit. Naming all fruits “devil fruits” would make tracking down THE devil fruit extremely difficult as any mention of THE devil fruit could be interpreted as any “devil fruit”. (including any poneglyphs or any other surviving documents or myths)
● Harahettania, Namakura Island (where Brook landed during the timeskip) was deemed as an important island by Kuma. This could have been to show that just like people worship Nika, there are some who worship the person who opposed him.
● I think that at the minimum Imu has fused with Satan, or whatever the highest ranking demon in One Piece would be. Which would explain the silhouette of their transformation when they were attacking Sabo.
● But, I also think that Imu can freely manipulate the demons they summon or have summoned. Similar to how Moria and Luffy possessed multiple shadows. This would allow Imu to freely transform into any demon in their arsenal which would make for an incredible final fight between them and Luffy, as both morph between different forms trying to counter each other’s attacks.
*Theory by KeyProposal3524
