Is there a belief that the seventh son of a seventh son is to hold or have a certain power that can inflict injury or harm onto any other person that is intentionally malicious to them?

Answer 1

There has been the superstition that the seventh son of a seventh son would become a werewolf. This superstition was particularly strong in Argentina where such a child usually 'came to harm'. To curb such atrocities a law was passed that such boys became the official wards of the President and became the recipients of benefits and scholarships. Since then the abuse of such boys has abated.