It's in Council of Thieves AP. Here is the preliminary:
Hellknights
The lawful orders of Hellknights are very much a part of
Westcrown. The closest Hellknight fortress, Citadel Rivad
(see page 10), houses the oldest of the Hellknight orders—
the Order of the Rack. During the Adventure Path, the
Order of the Rack may take on a relatively adversarial role,
and a PC who wants to become a Hellknight should not
associate himself with this order. The Pathfinder Chronicles
Campaign Setting provides more information about other
Hellknight orders, and Pathfinder Adventure Path volume
#27 will contain extensive information about these orders,
including a prestige class that characters can aspire to
take. In particular, the Order of the Scourge is an excellent
choice for a PC Helllknight, as this order is (among other
things) quite keen on opposing organized crime.
Although details of that prestige class are not presented
here, the requirements your character needs to fulfill before
taking the 15-level Hellknight prestige class are as follows:
Base Attack Bonus: +5
Weapon Proficiency: You must be proficient with all
martial weapons.
Armor Proficiency: You must be proficient with heavy
armor.
Alignment: Any lawful.
Special: You must defeat a devil as part of a special
ceremony witnessed by another Hellknight. More details
on this ritual are presented in Pathfinder Adventure Path
volume #27, but this information is not necessary for you
to worry about too much during character creation or your
first few levels of experience.
Yeah, get in early. The beast spans 15 levels!!!