My ex-paladin Fallen (#8159) has been blessed with a Dark Heart pact. This creates the ability to use DH + Hellfire when maybe it shouldn't. If it should, the MP cost is not correct. Only the -8MP from Hellfire is applying when the net MP from a successful hit should be +1.
Hellfire does not add any damage when used with DH though the status effect is still applied. I believe this part is working as intended.