Diabolist

Those wizards who follow the arcane tradition of Diabolism believe they are smart enough to cheat the devils themselves. In their personal arrogance they believe they can deal and bargain with fiends, gaining arcane power without suffering any dire consequences for their rewards. They take a shortcut to power, having enough arrogance to believe they can play with fire without getting burned. Where the Warlock is bound to, and becomes a "puppet" of a particular fiend, the Diabolist avoids such contracts and manipulates a devil's greed with their strength of will. They tend toward Lawful and Evil alignments.

Diabolical Power

Starting at 2nd level, the Diabolist has learned fiendish secrets to empower their spell attacks. Your spells critical hit on spell attack rolls of a 19 or 20. 

Diabolical Persuasion

Starting at 2nd level you are beguiling and persuasive as a devil. You gain proficiency in Persuasion, if you are not already.  In addition, for you, Persuasion becomes an Intelligence check rather than a Charisma check. 

Hellfire

At 6th level you add the faerie fire spell to your spellbook if it is not there already. You may cast faerie fire as a bonus action. In addition to the other effects of the spell, any creature that suffers from the effects of your faerie fire at the start of their turn also suffers 1d6 + INT modifier additional fire damage. 

Commune with Devils

Starting at 10th level, you may cast Commune. This does not expend a spell slot. While you will receive correct answers, in the case where a one-word answer might be misleading, or contrary to the devil's interests, the DM will most likely mislead you. Be prepared and carefully consider the wording of your questions. Once you use this feature, you cannot use it again until you finish a long rest. 

Summon Devil

Starting at 14th level, as an action, you may summon a devil of CR rating up to 1/2 your current level and charge it with a specific task. Once summoned the devil is allowed a CHA save against your Persuasion check to resist your command. If it resists it will act on its own free will, possibly even turning on the caster. Otherwise, it will try and fulfill its task within the next 24 hours before returning to the Nine Hells. Typical tasks involve killing a specific creature, guarding a particular object, or helping the caster reach a specific goal. Once you use use this feature you may not use it again until after a long rest.