In order to advance as a Hellknight, you must meet the following prerequisites (in addition to the multiclassing prerequisites for your existing class):

Hellknights gain thier knighthood after imbibing a special concoction of devil's blood, changing thier bodies into a terrifying fusion of the mortal and infernal. They train with summoned devils, the signifers of their orders coaxing the ingenuity of diabolical tactics from infernal tongues, while the rank and file members spar against the very denizens of Hell—making nearly any future conflict a far less daunting prospect. They learn that sacrifices must often be made for the greater good, obey draconian regimens of military conduct, commit to encyclopaedic memorization of the laws of their orders and local governing bodies, and undergo constant drills to train their bodies and minds.

The Chain: This philosophy encourages discipline through trial, whether such tests involve memorizing and meditating upon the Measure or exposing oneself to stress and pain that threaten to break the weak. At its more philosophical level, the Chain concerns itself with three virtues: order, discipline, and mercilessness.

The Measure: A massive codex of duties, laws, and crimes, the Measure serves as a guide to a strict, ordered society. Based upon centuries of legal codes, as well as passages from the strictures of Hell itself, this body of laws extols justness rather than justice.

The law and order the Hellknights live by and seek to impose are not the laws of any one nation. Rather, they are a set of strict philosophies known as the Measure and the Chain.

The Hellknights are a collection of knightly orders with a unique focus on the promulgation of law. They are interested less in the social goodness and charity typical of paladins, but rather in the foundation and stringent maintenance of order at all costs. In their iron-handed exaction of law, Hellknights emulate the organized and effective armies of Hell. They are not concerned with morality or methods, only results. If people cannot be trusted to obey the law out of their own senses of civility and social righteousness, Hellknights provide the need to obey out of fear of a master's stern hand. Regardless of their severity, Hellknights are not an inherently evil group; they are wholly unconcerned with morality. Although there are numerous evil members—particularly among their upper echelons—the majority of the orders are impartial arbiters and enforcers of order and justice. They see the study of Hell’s tenets and even the summoning of devils as tools meant to intimidate and strengthen the individual resolve of the orders’ members. Hellknights are taught to replace emotion with steely discipline, and are not interested in methods: only the end results matter.

Axiomatic Sense Much like a paladin's divine sensing ability, a Hellknight's infernal blood allows you to sense out beings of disorder. As an action, you can open your awareness to detect such forces. Until the end of your next turn, you know the location of any aberration, fiend, or fey within 60 feet of you that is not behind total cover. You know the type (Aberrations, fiends, or fey) of any being whose presence you sense, but not its identity. You can use this feature a number of times equal to your Hellknight level. When you finish a long rest, you regain all expended uses.

Requisition At 1st level, your Hellknight tutor presents you with your choice of one Hellknight requisition. Regardless of the choice you make, your requisition gives you +1 AC while you wear it. If you ever lose your requisition, you can replace it with 500gp and a weeks worth of work. Only you are proficient with this custom made requisition. Your choices are the following. Hellknight Plate. This plate armor does not give you disadvantage on stealth checks.

This plate armor does not give you disadvantage on stealth checks. Hellknight Mail. While wearing this half-plate, bludgeoning, piercing, and slashing damage that you take from non-magical weapons is reduced by 3.

While wearing this half-plate, bludgeoning, piercing, and slashing damage that you take from non-magical weapons is reduced by 3. Hellknight Leathers. While wearing this studded leather armor, you have advantage on Stealth checks.

While wearing this studded leather armor, you have advantage on Stealth checks. Hellknight Bracers. While wearing these bracers, your movement speed increases by 10 feet. These bracers do not count as armor.

Discern Lies At 2nd level, the infernal blood you've imbibed has begun to affect your mind and has heightened you awareness to lies. You gain advantage on Insight checks to determine whether a creature is lying. Additionally you have advantage on saving throws against being charmed.

Disciplines At 2nd level, you gain one discipline of your choice. When you gain certain Hellknight levels, you gain additional disciplines of your choice, as shown in the Disciplines Known column of the Hellknight table. Additionally, when you gain a level in this class, you can choose one of the disciplines you know and replace it with another discipline that you could learn at that level. Also at 2nd level you choose your Hellknight order, this order allows you certain disciplines that you wouldn't otherwise be able to choose, the order's and their tenets are listed at the end of this document.

Force of Will At 3rd level, the infernal blood has turned your mind into iron. You have advantage on all Intelligence, Wisdom, and Charisma saving throws made against any spells or abilities used by Aberrations, Fey, and Fiends.

Lawbringer At 3rd level, you have mastered the laws of The Measure and you have a vast knowledge of the laws of the lands. You gain advantage on all Intelligence, Wisdom, and Charisma based checks pertaining to law, lawkeepers, and criminals. Additionally, learn Infernal if you do not already know it.

Aura of Law At 4th level, you radiate an aura of lawfulness that causes lawful beings innately trust you even if it's against their best interests. Devils, Modrons, and other lawful creatures that would be indifferent toward you instead are friendly to you. If they would be hostile they are instead indifferent. A creature is only effected by this feature the first time they see you.

The Blood and The Chain At 4th level, you have become a paragon of The Chain and you can refresh your mind by reciting its virtues. At the end of a long rest, you can choose to increase one ability score of your ability score by by 2 or increase two ability scores of you choice by 1. At the end of of each subsequent long rest, you can alter this choice, reducing the scores you previously increased and improving different scores. (If you chose to increase two abilities, you can opt to alter only one of those choices.)

Infernal Armor At 5th level, you have mastered the use of your Hellknight requisition and you gain an additional benefits while wearing it. Choose on of the following features of your choice. Resilience Against Cold: You have immunity to cold damage and magical effects that would reduce your movement. In addition, you and everything you wear and carry are unharmed by cold temperatures.

You have immunity to cold damage and magical effects that would reduce your movement. In addition, you and everything you wear and carry are unharmed by cold temperatures. Resilience Against Fire: You have immunity to fire damage. In addition, you and everything you wear and carry are unharmed by hot temperatures.

You have immunity to fire damage. In addition, you and everything you wear and carry are unharmed by hot temperatures. Resilience Against Poison: You have immunity to poison damage, the poisoned condition, and disease.