School of Demonology

You focus on the aspects of binding outsiders to your will. You delve into dangerous magical arts, prying ancient secrets from those who torment the material plane, learning the true names of devils, demons, and other fiends and striking pacts with such creatures. You also practice a balanced art of self-preservation, learning arcane arts to embolden your defenses to such outsiders. Due to the nature of arcane methods, you draw power from your force of will, rather then ineffable mind. Akin to a warlock, your power comes from Charisma, and your ability to strike deals.

Demonologist

Beginning when you select this school at 2nd level, whenever you conjure or otherwise summon a fiend, if the creature would normally be hostile to you, it is not hostile to you or a number of allies chosen at the time of casting. You may select a number of allies equal to your charisma modifier. If the creature would break free from your control, it only becomes hostile to you if you actively attack or otherwise hinder it.

Bound Servant

You learn the find familiar spell and can cast it as a ritual. The spell doesn’t count against your number of spells known.

When you cast the spell, you can only choose one of the following special forms: imp or quasit.

Additionally, when you take the Attack action, you can forgo one of your own attacks to allow your familiar to make one attack with its reaction.

Embolden Pacts

Starting at 6th level, when you summon, conjure, or call a fiend that you can exercise control over, you empower that creature. That creature adds your proficiency bonus to its attack and damage rolls. In addition, it's maximum hit points are increased by an amount equal to your class level.

Fiendish Servant

Beginning at 10th level, you have discovered a method to bind a fiend to your service. You may perform a special ritual taking 8 hours. When you perform this ritual, choose a fiend whose CR does not exceed one half your level, rounded down. As the ritual completes, the fiend appears in the specially prepared summoning circle. You then spend the next hour forming a pact with the fiend, promising it a specific reward for it's service, payed within the next year. Appropriate rewards for services is the soul of a creature whose CR equals or exceeds the fiend's CR, a donation of wealth in the form of magic items, appropriate in rarity to the fiend (uncommon for CR 1-3, rare for CR 4-6, epic for CR 7-9, and legendary for CR 10) or similar payment, at DM's discretion. The being stays for a year and a day, after which it returns to its home plane, taking it's payment with it. If the fiend would die while in your service, the magic of the pact ferries it away tot the astral play, wherein you may spend another 8 hours performing a ritual to reform it on the material plane.

A fiend bound to your service in this manner serves you as best as possible, lending you reasonable aid when necessary, and heeding your instructions in combat. 

When the year of service is complete, if you are unable or unwilling to relinquish payment for services, your soul is forfeit to the fiend. If your soul is already forfeit to another being, or you are without a soul, the fiend instead takes your life, binding you to it's service for a year and a day before relinquishing your body and life to the void.

Twisted Conjurations

Starting at 14th level, whenever you cast a spell that conjures, summons or calls one or more creatures, those creatures are treated as a fiends for the purposes of your class features. In addition, your appearance takes on a fiendish quality from your prolonged exposure to fiendish energies. You have advantage on all charisma checks involving fiends, and fiends have disadvantage on charisma checks involving you. Whenever you cast a spell that would conjure, summon, or call a fiend, you may have that fiend's maximum hit points increase by an amount equal to your level.