Hellfire of Mephistopheles is a paragon tier feat.[PHRT:28]
Prerequisite: 11th Level, Tiefling
Benefit: Each time you hit an enemy that has fire resistance, after resolving the attack, reduce the value of the resistance by 5 until the end of the encounter. If the target has no fire resistance (or its resist value has been reduced to 0 or lower), it instead gains vulnerable 5 fire.