Got to say, this is probably one of the best homebrew martials I seen for 5.5e! The simplicity of its gameplay loop similar to Rogue (Action Attack, Bonus Action to Strike Fear, Repeat) is simple yet effective.
Not only that, but also the fact that this also provides fun roleplaying aesthetic as an edgy dark knight who protect their friends is one of my favorite tropes! Also bonus points for having more then just 4 subclasses but also feats, new weapons, and magic items!