Soulfire uses part of the human soul to enhance the user's magic. The soul regenerates over time and through acts which are "good for the soul", but will kill the user if it is completely drained.
Soulfire gives structure to magic. For example, it could theoretically allow a path to be made into deeper Faerie by stabilising the gateway. While angels use this, it is unknown if they use it on its own or bound to a form of magic.
Despite being similar to Hellfire in enhancing magic, there is a key difference: Hellfire gives power to the means; Soulfire gives power to the intention .