Hellfire Conduit

Infernal power flows through your veins allowing for your flames to burn even hotter, for the price of your physical constitution. When you cast a spell that does fire damage you may expend a number of hit die up to the level of the spell to add extra damage to the spell. This hellfire adds unique properties to the fire associated with the spell:

• Spells imbued with hellfire ignore creature resistance and immunity to fire damage. Devils are immune to this effect.
• Hellfire cannot be extinguished by water
• As an additional cost to the spell you must resist the physical exhaustion of empowering the spell with hellfire. Make a constitution saving throw (DC of 10 + spell level) or suffer one point of exhaustion.