Imu is the king, “Mu” is the entity.
From the mural, it shows a king approaching the devil creature in the tree in the 1st World.
This appears to have preceded devil fruits, so that devil entity itself with the horns and wings either possessed the human king or survived into the 2nd world on its own.
Imu still has human concerns and mannerisms, so it does seem to be a case of possession. The entity’s powers would be like Poseidon or the devil fruit tree, “magic” independent of devil fruits.
The “unearthly” nature of the Gorosei & Shamrock’s sword and the nature of the immortality he can bestow indicates that it is not devil fruit-derived.
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One more point against it being a devil fruit is that it would be included in the encyclopedia.
It would also make the Op Op fruit redundant and be worth insanely more to be able to make multiple people immortal as well