this scale belonged to kaido, a famed warrior who could never be defeated and a tamer of white-eyed blue dragons. with such a resistant shield, you'll have a hard time losing hp. if you carry it, only a being of divine power will be able to inflict damage on you...

as for kaido, no one knows what became of him. legend has it he no longer needed a shield to protect himself, which wouldn't be a surprise, given his skill in combat - even magmog was no match for him.