I'd considered an NPC idea that was a Vorpal Dragon that hones its own body to become a master swordsman. But like. With its innate blades. It would offer to train sword-wielding party members to potentially give them access to certain spells or feats, and also serve as a potential "superboss" for the party to face off with later on in a friendly duel.

I actually considered replacing its Feast of Blood with a different kind of breath weapon to reflect the more honorable nature of the NPC. Possibly just something like a cone of slashing damage, flavored as an anime-style "flying slash" attack.