Around the storyline of my abyss sea au, silvy’s body was burning inside out, due to absorbing so much magic energy while in iron skin 5, and when it ran out, he started burning and they had to give him a dose of iron skin 5, which kept him alive but it was also limited, so his people desided to try and make a cure for him.
He wanted to die in peace, but one of them still gave him he serum, the parts of his body that were already burnt away were most susceptible to the change, his entire dna and humanity was changed.
His personality, instincts, nature and body were changed.