It's not a formal title, just a descriptor used by followers of Asyra, Bim-Shalla, and the Dawnbreaker to describe those who have dedicated their life to driving back the forces of darkness. It's generally used to describe knights and paladins, and other soldiers who have made a mark against evil.
Vorpal swords were magically forged and enchanted by human mage-smiths during the war between the Empires of Frelhal and Sen Ahreal, but after the cataclysmic destruction wrought by the ensuing mage battles and the annihilation of Frelhal, the secret of their making was lost.
Today, there are a smattering of the ancient blades across Etheria, all highly treasured.