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After Kireina was asked by Hydros to be given a physical body reminiscent of mortals, she came out with several ideas… There were many ways for her to grant a physical body to a god at this point, which could be called something like Reincarnation, in a way.
However, such a thing could only be practiced in exceptional cases such as Hydros, Megusan was also an exceptional case who did it to himself willingly. Megusan seemed to have used to be a male demon demigod, but after having its physical body destroyed and its divine core stolen, it was split into pieces and weakened tremendously, making it easy for him to see physical mortal vessels as good opportunities to ascend to godhood once again.
Megusan had been eaten by around 99%, leaving a small split soul inside an egg laid by his strongest physical vessel, the Giant Venomous Snake Lamia, this egg was where Nirah was then born.