Blackflame Mastery

Damage Resistance. You gain Resistance to either Fire or Necrotic damage (chosen when you take this feat).

Blackfire Witchcraft. You can choose for any Fire damage you deal to be Necrotic, and vice-versa. Immediately after you deal Fire or Necrotic damage to a creature, you can choose as a Bonus Action to begin Concentrating. The creature makes a Charisma Saving Throw (DC 8 + Spellcasting or Constitution Modifier + Proficiency Bonus). On a failure, it gains the Burning (1d10) condition and can't be doused for as long as you maintain Concentration. While Burning in this way, the creature can't regain Hit Points.

Note: Due to DDB Limitations, the choice selector will give you the ability to resist any damage type of your choosing. Picking a damage type that is not Fire or Necrotic damage is outside the scope of this feat and considered cheating.