This Arcane Tradition has it’s roots in the old White Dwarf class of the Summoner, these ended up becoming a set of very powerful mages in the Shadowlands who essentially acted, as one NPC put it, “lawyers”. In truth, you could also have looked as them as almost a sort of rabbi, or a student of the Kaballah, who studied the supernatural laws of the multiverse as well as the pacts and bindings of all supernatural creatures so that they could effectively advocate on the behalf of themselves and others with the various Powers.
In any case, I decided to see what I could do about translating them into an Arcane Tradition rather than making them an entirely new character class. This is much the same as I did with the College of Tantra for the Bards which were based on the old Houri class from White Dwarf. It also includes nods to a handful of other sources that have informed the play of the class over time.
The study of Diabolism is most popular in the Shadowlands, or in groups and families that have strong connections (usually positive but also negative) with the Fey, Fiends , Elementals, and even Celestials. Similar in minor ways to a Warlock, a Diabolist (as those who study Diabolism) has an encyclopedic knowledge of spirits and the various covenants, pacts, and agreements that bind them – some call them the “lawyer’s of the shadowlands”. As a wizard who studies the School of Diabolism, you favor spells that summon creatures and spirits, ritual magic, and the magic that binds such creatures into service.
In the Shadowlands it is quite common to have a Diabolist on hand for any negotiations with appropriate creatures, as well as being a resource for obscure lore on the spirits being spoken with. Often a Diabolist is the only one with the knowledge of the correct protocols, modes of address, and aims and desires of said spirits that even makes such negotiations possible.
At 2nd level, when you select this school you invest a shortsword, longsword, or greatsword with magical energy, are granted proficiency with it (and it alone, not with all weapons of the same type), and may use it as an Arcane Focus. The sword is also treated as magical for damage effects, and the Diabolist may use their Charisma as the modifying statistic rather than Dexterity or Strength if they so wish. Only one sword may be invested at a time, all previous investments are lost upon making a new Oathbound Sword.
Starting at 2nd level, Diabolists are able to call upon the Shadow Proclamation when dealing with summoned creatures and take a neutral position of parley and negotiation with the summoned creature. While the Diabolist cannot cause the summoned creature to break their oaths or bindings, they will usually give the Diabolist (perhaps only the Diabolist if they are part of a group) the chance to retreat rather than attack them out of hand. This is instantly nullified if the Diabolist attacks the summoned creature or attempts to thwart their oathbound mission.
Greater Heptogramic Warding
Upon reaching 6th level, the Diabolist learns the secret of the Greater Heptogram, a seven-pointed symbol that greatly enhances the efficacy of abjuration spells against Celestials, Fey, Fiends, Elementals, and the like (this includes Dearth and Undead). The Diabolist may add the Charisma modifier to the Spell Combat or Spell DC of their abjuration spells against such foes.
Seven Dread Phrases
Diabolists of the 10th Level have knowledge of the Seven Dread Phrases, which allow them great power over creatures originating from or with strong ties beyond the Mortal Realms. By uttering one of the phrases, the Diabolist can beak the Fear, Charm, or Possession of any single target within 10 feet by one of those creatures automatically. Alternately, as an action, the Diabolist can target any single of the affected supernatural creatures within 10 feet and attempt to drive the creature back to it’s home plane. The creature must make a Saving Throw (Charisma) or be immediately returned to their native Realm. This use causes 1d6 HP of damage to the Diabolist. Upon using any of the Seven Dread Phrases, they may not attempt this again until after a Short or Long rest.
At 14th level Diabolists learn the secret of the Eschaton Sanction, a pact so old and so powerful it has a ruinous effects upon all who hear it within 30 feet. Each creature must make a saving throw (Charisma) and if failed the following effects occur, based on their current hit points.
50 Hit Points or Less: Deafened for one minute.
40 Hit Points or Less: Deafened and Blinded for ten minutes.
30 Hit Points or Less: Deafened, Blinded, and Stunned for one hour.
20 Hit Points or Less: Killed Instantly.
In any case, all creatures from other than the Mortal Realm that fail their save are instantly forced back their native Realm and cannot return to the Diabolist’s current Realm for 24 hours by any means short of a Wish. As a final effect, all creatures of the Dearth take 10d6 Psychic Damage (save for half damage) if subjected to this spell, after which the primary effects are determined.