The military outfit is behind the Mourningwood Demon Door. The door requires that you have unlocked all the augments on a legendary weapon and also have maxed out your skill relative to that weapon. For instance, if you have a fully upgraded Briar's Blaster you will need to have unlocked the final ranged weapon skill chest on Road to Rule. That being said, you can often trade for it using your king/queen outfit, masquerade outfit, or various weapons.
Anything besides your hero weapon is typically considered a "legendary" weapon, depending on who you ask. Your hero weapons will not work, is what I meant.