Comments from the Finger: This is an oft' requested item that draws inspiration from the Hellfire Warlock of 3.5
You are descended from fiends of the Lower Planes, eternal enemies of light and good. The traits have shown up in your bloodline for generations: hot blood, habitual cruelty, and bone spurs of horns barely visible on the head, but you were born with a stronger connection. You can summon unearthly fire at your fingertips that burns most anything, and you feel the constant urge to scorch those who stand in your way. And, of course, like your ancestors before you, you lust for power and to corrupt the souls of the innocent.
At 1st level, you have a devil's silver tongue and sharp wit, which you find is best used to despoil the innocent. You have advantage on any check you make to negotiate the terms of a deal or write a binding contract in your favor.
Infernal flames burn in your blood. At 1st level, whenever you deal damage with a sorcerer spell, you can change the damage type to fire damage. Also, when you deal fire damage, you ignore damage resistance,
Starting at 6th level, you can use your reaction when you take damage to spend one sorcery point and cast the spell hellish rebuke without expending a spell slot.
By 14th level, you are more like your fiendish ancestors than anyone else in your lineage. You have resistance to fire and cold damage. Additionally, you can use your bonus action to spend 2 sorcery points and gain resistance to bludgeoning, piercing, or slashing damage from nonmagical weapons that aren't silvered until the beginning of your next turn.
At 18th level, you fire burns hotter than any originating on the mortal planes. When you roll fire damage, you deal an additional point of damage for each die rolled. Additionally, if you deal damage to a creature that is immune to fire damage, it is treated as only having resistance against it. You do not ignore this resistance.
Changelog: 10/17/16: Typos
10/18/16: Tiefling Requirement cut
True Hellfire: Does not also ignore resistance
10/26/16: Descendant Fiend: Cost reduced to 2
11/13/16: Descendant Fiend: Offers fire resistance as well