Fiendologist

A wizard who specializes in summoning demons and making deals with devils.

Fiendish Tongue

Beginning when you select this school at 2nd level, you gain the ability to speak, read, and write in Abyssal and Infernal. You also gain advantage on Charisma(Persuasion) and Charisma(Deception) checks to interact with fiends.

Dark Knowledge

Beginning at 2nd level, you learn the true name of a Dretch, Imp, and Quasit and you gain advantage on Wisdom(Insight) checks against fiends whose true names you know.

Forbidden Rituals

Starting at 6th level, you add the Magic Circle spell to your spellbook and you always have it prepared. You also learn the true names of a Bearded Devil, Hell Hound, Nightmare, and Succubus/Incubus. When you cast Magic Circle, you can choose to perform a fiend summoning ritual. The summoning takes 5 minutes and a vial of fresh blood to complete. Once the ritual is complete you summon a fiend whose true name you know inside or outside of the circle. While the fiend is summoned it is less overtly hostile to you, unless the Magic Circle spell falters, was done incorrectly, or it convinces you to let it out or to enter. You become well versed in fiendish pacts and can attempt to form one with a fiend summoned this way, usually for a service or knowledge. The fiend will try to take advantage of the wording of the pact and will usually demand something of equal or greater value, and can ask for things like emotions, years of your life, or your soul after a set time has passed. Some pacts can be one time deals such as a retrieval of an item, or some can be more long term, like the power and guarantee to summon the fiend to aid you in battle. Devils are more willing to make deals but are more cunning, while demons are less likely to make a deal but usually stupider. Especially stupid or bestial fiends will often make deals without trying to trick the other party.

Well-versed Pact-maker

Beginning at 10th level, you become well versed in making pacts with fiends, and you add your Intelligence modifier to any Charisma(Persuasion), Charisma(Deception), or Wisdom(Insight) check made to interact with fiends. You also learn the true name of a Barbed Devil and a Vrock.

Master of Demons

Starting at 14th level, you learn the true names of a Hezrou, Chain Devil, Glabrezu, Bone Devil, and a Horned Devil. Whenever you make a pact with a fiend whose challenge rating less than or equal to half your wizard level(rounded down), you can trick it into making a pact where it doesn't get anything from you or any benefit. After making the pact the fiend will realize what has occurred and will swear to get revenge if it ever gets free, but will follow through with the pact to completion, though it will do so with extreme spite and will take advantage of any weakness or vague wording. In addition to this you also become immune to being frightened or charmed from dealing with fiends so much.