Hellfire

Hellfire is a demonic magical power boost to destructive spells; it is the antithesis of Soulfire.

Description
Harry Dresden gains hellfire from Lasciel's Shadow, while she is in his mind. Using it has a characteristic smell of sulfur and brimstone.

The sigils on Dresden's staff and blasting rod light up with a reddish glow when Hellfire is called forth into a spell, while a shimmering wave of force flows out from him.

Dresden usually feels exhausted after the use of hellfire. It seems to encourage Lasciel to insert herself more strongly into his consciousness—Lash was unable to communicate with Dresden until he made the conscious choice to use it, likely due to his free will. Lash appears to him in a dream immediately after his fight with Cowl (the first time he uses Hellfire).

Comparison to Soulfire
The key difference between soulfire and hellfire, is that the first powers the intention, the second powers the means.

If Dresden uses a spell meant to send a wave of force and knock someone down, Hellfire makes it stronger. On the other hand, Soulfire makes a silvery hand that Dresden can control with the same power of the original spell but given more focus.

Blood Rites
In Blood Rites,

Dead Beat
In Dead Beat, Harry Dresden draws on Hellfire to empower his fight against Cowl, throwing a car on top of Cowl.

Proven Guilty
In Proven Guilty, Harry Dresden uses Hellfire fighting the Scarecrow at the Full Moon Garage after escaping Darby Crane who kidnapped him. The Hellfire has no effect on the Scarecrow.

When Dresden and his team enter Arctis Tor, they find hundreds of dead trolls and goblins, the air full with the scent of sulfur and brimstone, their bones cleaned, the flesh consumed by what was likely Hellfire. Harry uses Hellfire in battling Fetches that attack them.

White Night
In White Night, Harry Dresden subsequently loses the use of Hellfire after Lasciel is erased from his mind.

Small Favor
In White Night, at the Shedd Aquarium, Harry Dresden confronts Nicodemus Archleone about the fire used in Arctis Tor, and that one of his Denarians was in on the Black Council raid. Nicodemus did not react well to the information. Calming, he wonders whether the contamination is among standing members of his order. At the end, Dresden learned the Denarian that used Hellfire at Arctis Tor was Thorned Namshiel.

Uriel bestows upon Dresden the angelic counterpart to Hellfire, Soulfire