[Chosen Infernal at Heart] Those that are Abyssal have achieved a closer attunement with their patrons, further than the surface reaches of an Infernal might. The Abyssal is considered a higher tier of Fiendish, but does not have the abilities the Infernal have, including dark vision, mimicry, etc. The difference between the two is that those that are Abyssal are better at controlling their emotions. Some are even able to supress their influence so that it becomes miniscule. However, those Abyssal can taint objects and turn Infernal themselves if they choose to. They can then consider themselves [Abyssal + Infernal] or "Abyssal & Infernal at Heart". During the act of making a Contract, they can also gain the additional Infernal trait as part of their bargain. They will then carry all abilities the Infernal have. The lesser Fiend cannot be unbound unless the Abyssal is severed from their contract or has died and revived, in which they return to being just Abyssal.
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There's been a lot of questions and confusion about differences between Infernal and Abyssal. I hope this clears things up as I've decided they're two seperate distinct paths. But, if you're Abyssal, you can be Infernal as well.