Soulfire consumes part of the user's soul to enhance their magic. The soul regenerates over time, or through acts which are "good for the soul", but will kill the user if it is completely drained. While Hellfire increases the power of violent and destructive spells, Soulfire increases the power of creative spells, using a portion of the soul as a matrix for a spell in the process.
Soulfire gives structure to magic; it could theoretically allow a path to be made into deeper Faerie by stabilizing the gateway.(reference needed)Angels use Soulfire, as they are made of nothing but soul.
In the seriesEdit
In Small Favor, Harry Dresden first unknowingly uses Soulfire against Namshiel in the Aquarium. It manifests itself as a massive silvery simulacrum of his hand. Dresden is granted the use of Soulfire by the Archangel Uriel.
Bob informs Dresden that Soulfire is powered by one's soul. It forms a matrix around which one builds a spell, giving it much more power. Given time, the soul can regenerate, unless it is used all up. If the soul is exhausted, however, the consequences are not pleasant.
During his battle on Demonreach with Shagnasty the Naagloshii, he again uses soulfire to super-charge his supernatural garrote. Though he defeats the spell by making a Circle of Power, Shagnasty seems impressed with Dresden's use of it, asking who gifted him with the "life fire". As the creature approaches to kill him, Dresden starts to prepare a Death curse backed with soulfire. He isn't sure if anyone has ever done so before, or what the consequences would be, but he is saved the trouble thanks to Toot-Toot's rescue.
In Cold Days, Harry Dresden understands that soulfire is fueled from the user's entire being - not only from the 'soul' but the amalgamation of what makes the Self (a product of every decision made, every thought we have, every action done), and that it is the user's entire being and identity that fuels it.(reference needed)