Hellfire

Hellfire is a demonic magical power boost to fire and destructive spells.

In the series
Harry Dresden gains this power from Lasciel's Shadow, while she is within his subconscious mind, and subsequently loses it after she is erased from his subconscious at the end of White Night. Harry is later given the angelic counterpart to Hellfire, Soulfire, by the Archangel Uriel somewhere in the middle of Small Favor. Workings in Hellfire have a characteristic smell of sulfur and brimstone

Despite being similar to soulfire in enhancing magic, there is a key difference: Hellfire gives power to the means; Soulfire gives power to the intention. If Harry uses a spell meant to send a wave of force and knock someone down, Hellfire has made his wave of force stronger. On the other hand, Soulfire made a silvery hand that Harry could control, with the same power of the original spell but given direction like a hand (rather than the simple 'push' that it gave before).