Conduit of Hellish Fire

The infernal being to whom you have sold your soul has granted you access to the fires of Hell. Like everything else with such bargains, this comes with a price. A number of time per day equal to your Charisma bonus (minimum of 1), you may channel the fires of Hell. You can do this for either attack or defense. When making a spell attack, you infuse the magical energy of your attack with hellfire, doing additional 1d8 fire damage. As a bonus action, you can wreath your body in hellfire, giving you resistance to cold damage and immunity to fire damage until the start of your next turn, and increasing your armor class by an amount equal to you Charisma bonus.  You can chose either the attack option, the defense option, or both on your turn. Each use of this feat, does 1d4 damage to you, which nothing can prevent or reduce.