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 SoulfireEdit
If Harry 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.(reference needed)
In the seriesEdit
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. Dresden uses Hellfire in battling Fetches that attack them.
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.