Corinnguard
Hero
Gotcha. Or the average homebrewer.Up to the Narrator.
Each Mythical Martial tradition comes with the caveat that a Narrator has to decide whether someone starts proficient with it automatically. But also provides an organization that can teach you to use it, and a magical item that allows you to learn. And, of course, the Narrator can always add in their own setting-specific ways to become proficient.
So there's multiple ways to access it, but it's going to depend on your narrator.