Demonology

Arcane Tradition

A demonologist is a mortal who has devoted his life to the study of demons. He is practiced in dealing with them, fighting them, and using them for his own gain. He even gains some demon abilities due to his studies and devotion to the fiends of the Abyss. Demonologists are always evil, although they might not think of themselves as such. No character can get so close to the mind of utter corruption and come away unchanged.

Demonologists see themselves as researchers and keepers of lore that most people cannot handle. Thus, they often sequester themselves away in libraries of dark knowledge. They attempt to deal with demons on an equal footing—an extremely dangerous undertaking—and rarely interact with mortals. When demonologists see the need, they emerge from their studies and use their dark powers to take or do what they want; then they return to their books and summoning circles. PCs might come into conflict with a demonologist who is after some rare bit of lore or a magic item that they possess. Or the characters might need the help of a demonologist if they are planning to fight demons and need aid or information.

Demon lore

Starting at 2nd level, your extensive studies of demonkind and the Abyss have gifted you with forbidden knowledge. You add your proficiency bonus to any check to recall information about demons or interact with them. If you would already add your proficiency bonus to such a check, your proficiency bonus is doubled.

You learn the Abyssal language as well.

Demonic Familiar

At 2nd level, you add the spell find familiar to your spellbook. Whenever you cast this spell, your familiar will always be a quasit. If you already have a familiar when you gain this ability, a quasit appears after a long rest and devours it, replacing it as your familiar. The quasit cannot end its service to you for any reason.

Abyssal Resistance

At 6th level, your exposure to the Abyss has given you a natural resilience to its influence. You have immunity to poison damage, as well as advantage on saving throws to resist the effects of Insanity.

Summon Demon

At 10th level, you add the summon greater demon spell to your spellbook if you don't already have it. When you cast this spell, you can use your own blood as a conduit for the spell. Instead of the material component, you can inflict 1d10 slashing damage on yourself with a dagger or similar weapon as part of the casting of the spell. If you do so, you can ignore the material component of the spell.

Dominate Demon

At 14th level, you can cast the spell dominate monster targetting a Fiend(Demon) you can see without expending a spell slot. The creature does not gain advantage on the saving throw against this spell using its Spell Resistance if it has such an ability. You regain use of this ability after a long rest.

Credit: Manthos Lappas