As a Game Master who’s been involved in games in the Golarion setting for at least four years, I have always found the concept of Hellknights fascinating. Oppressive knights of order who meet out justice according to the letter of the law. The rules for playing a Hellknight were published in Pathfinder Adventure Path #27 “What Lies in Dust” and present the Hellknights as a prestige class.
Prestige Classes, however, are clearly against the design philosophy of the Pathfinder Roleplaying System. Instead Pathfinder presents two ways to vary core classes within the rules to better customize them to your character concept: those two customizable options are customizable options inherent in the character class itself (orders for Cavaliers, bloodlines for Sorcerers, etc) and Archetypes. Between these two rule features, one should be able to customize one of the core classes to be whatever one wants to represent- in this case, the Hellknight.
Unfortunately, none of the orders really strike me as appropriate to Hellknights. So I came up with this “Order of the Hellknight” for my own game. I am presenting it here for feedback and general consumption. I took one liberty with this and actually changed one of the features of the base class- that of the Banner. This would technically make this an Archetype. But you can simply disregard this switch and use the Order of the Hellknight as you would any other order and it still works fine.