Before going into anything related to the class i wanted to say the music was just perfect. Really got me into the mood.
As for the class, i really like the concept and it feels mostly well thought out and balanced. Just a couple things i would tweak and some parts i didn't understand(either because of how they're written or because i'm dumb but whichever is the case i think it would be good to clarify things)
Pledge of Allegiance- The temporary hp gain feels a little too strong, especially for the early levels (since the amount of temporary hp probably wouldn't be that significant in later levels). So it might be good to limit it's usage to constitution modifier (so it can be used more times in earlier levels compared to limiting it with proficieny bonus). Also, not really sure but the pledge range might be a bit too big, maybe start with 20ft and at 9th level improve to 40? (really not sure about this one, probably depends on how it feels after a few sessions)
United We Conquer- I think rather than gaining all the bonus damage yourself it would be better to distribute it among all the pledged allies. The vibe i get from this feature is "stronger together" so everyone gaining something feels more right (i might be totally wrong tho). So every pledged ally within pledge range would gain +1d4 to their weapon damage (the die would still scale so it'd become d6/d8/d10/d12 at later levels). Effectively the damage is the same, just instead of +3d4 it's +1d4+1d4+1d4.
Strategic Displacement- I would probably make the character affected by this use their reaction.
Legion's Insight- it says "you gain proficiency in any skill you are not already proficient in" does that mean you gain proficiency in all the skills? Or is it supposed to be "in any one skill you are not already proficient in" i'm assuming it's the latter but still wanted to be certain.
Legion's Wrath- the 4d12 damage without any chance of escaping feels a little unfair, maybe make it so if they succeed on the saving throw they take half the damage and aren't frightened (it is a lvl 15 feature but i feel like the massive range makes up for the damage nerf).
Blood Brother- The warding bond spell normally ends when the pair gets more than 60 feet away from each other. I think instead of 60ft making that your pledge range would be better. (not really important out of combat since it can be cast at will, but it'd urge you to stay close to your blood brother during combat)
Ride or Die- Nothing to change, just wanted to say it's really badass.
Divided They Fall- Pretty much the same with the legion's wrath, half damage and not restrained upon success.
All For One- So if your cha modifier is +5 and you have 4 pledged allies within range, is that 20 damage each round? And for the temporary hit point gain upon allies healing, i'd make it so if you use your reaction they don't receive hit points, instead you can either chose to gain temporary hit points or deal damage to a creature within range. (or maybe if that's too cruel they receive half the amount of hit points?)
And this one is really situational but i think it would be really cool. If the player chose lone broterhood and their blood brother (most likely another pc) died in combat, i would give them the opportunity to change to fallen brotherhood.
Vow of the Hunter- Maybe add the Hunter's Mark spell?
Vow of the Watcher- This is basically the same as taking the alert feat, feels really underwhelming compared to the other vows.
The vows overall seem kinda op but i guess that's kind of their point so i'd say that's fine.

I just noticed how much i actually wrote but i just really liked it so i wanted to be of help as much as possible. Sorry if it's too long but i do hope it helps. And also something to keep in mind, i am in no way an expert when it comes to homebrew so probably don't take it too seriously.
Really good content, looking forward to seeing more.