We’ve discussed the lore behind Hell Knights. But what are they mechanically?
They are a prestige class - you have to meet a number of requirements: be lawful, have a base attack bonus of at least +5, intimidation 5 and knowledge (planes) 2, proficient with heavy armor and slay a devil with Hit Dice greater than yours (should be witnessed by Hellknight). Easiest way is to take Armiger archetype for a Fighter - it puts you on the path to become a Hell Knight with some straightforward features (like additional skill points for Intimidate and Knowledge).