From publisher blurb:
Some evils are far more insidious than others - it's easy to point to a rampaging beast and declare it a threat to the people, but truly monstrous evils hide behind authority and legitimacy, controlling the innocent and ignorant from the shadows. These outspoken demagogues offer both safety and tyranny in the same hand. Few will challenge them, and none of those challengers' reputations will survive. For these reasons, you have donned your mask, and will stop at nothing to force tyrants to remove theirs. These villains may declare you a scofflaw or a vigilante, but that just demonstrates their fear of someone willing to stand against them.
Bards take on the mantle of the College of Justice for a variety of reasons, typically in the face of a personal tragedy such as the loss of a loved one or recognition of some other failure of the society in which they live. Drawing strength and daring from their anonymity, these bards may go to extreme lengths to right the wrongs of their society, breaking any law they feel has become a barrier to their cause. These bards aren't unethical, however - each must decide for themselves the lines they'll draw between right and wrong, and may often find themselves regularly forced to confront the consequences of these decisions and test the limits of their moral resolve.
Expanding upon the mechanics of the 5E Bard class, this PDF allows you to create mysterious vigilante bards using playtest-draft rules.