This is the thing used by the Undead to move their bodies and summon more of their kind using Soul Magic.[1] Because of this, undead can usually read another's soul fire as a way of determining whether they are an enemy.[2]
The undead are also able to use a type of icy blue soul fire to attack others.[2]
The talent, Unyielding, is able to ignite a living person's soul fire in order to survive otherwise fatal injuries.[2]