The answer so far — best supported
Chapter 1086 supplies the name, chapter 761 supplies a plausible route to immortality, and the Empty Throne supplies the motive for eight centuries of secrecy.
Imu Is Nerona Imu, the Immortal Survivor of the First Twenty
59% confidence · 2 pieces of evidence
Where it happens
8 chapters · first raised in ch. 603Theories in the race
All races, devil fruits and *all* branches of haki are encompassed under the umbrella of lineage factor. The factor appealing to most would be the implication that conquerors haki is also passed through genetics, like armament and observation. Gaining the ability to manipulate lineage factor is said to be "stepping into the realm of God". God, Imu, may have abilities attributed to the changing of his own/others lineage factors to gain stronger bodies and abilities.
The empty throne's introduction scene could be a key piece of foreshadowing.
Chapter 1086 supplies the name, chapter 761 supplies a plausible route to immortality, and the Empty Throne supplies the motive for eight centuries of secrecy.
Imu is able to access the devil fruits, haki, experience, and racial abilities of each of the different founding kings.