When you gain access to a new fiendish mastery, choose one of the following. Some masteries have requirements, such as minimum diabolist level or another mastery. You must meet those requirements before you choose that mastery.
Prerequisite: 6th level
You can cast the blood-writ bargain spell once per long rest without using a spell slot.
When making an ability check to buy, sell, or trade you can choose to gain a 1d6 expertise die. After using this feature a number of times equal to your proficiency bonus you can’t use it again until you finish a long rest.
You are a consummate liar. You gain proficiency in Deception, or if you are already proficient you gain an expertise die. Your maximum expertise die size in Deception is a d12, exceeding the usual limit.
In addition, you gain two Deception skill specialties.
Prerequisites: Binding Contract, 10th level
You can cast the geas spell once per long rest without using a spell slot.
So much bargaining with creatures from beyond has made you an old hand at negotiating with them. You gain an expertise die on Deception, Insight, Intimidation, and Persuasion checks made against creatures that have an alignment trait.
You can use an action to command your familiar to trail a creature you can see. While trailing the creature your familiar remains within 60 feet but no closer, it is invisible, can move through creatures and objects (it takes 1d10 force damage if it ends its turn inside an object), and you can telepathically communicate with it as long as you are both on the same plane of existence.
You can use an action to cloak your thrall in a guise of your choosing (as disguise self), either as a beast or humanoid depending on its form. While cloaked you are able to telepathically communicate with your thrall as long as you are on the same plane of existence and you can sense its surroundings (as the find familiar spell)
You can understand all the major languages of malevolent beings. You learn whichever two of the following languages are not covered by your diabolist archetype: Abyssal, Deep Speech, and Infernal. In addition, you gain proficiency in Persuasion, or if you are already proficient you gain an expertise die.