The Killing Fog by Jeff Wheeler

A fantasy in an Asian-like land where magic is real and when it is used, it draws the killing fog.  A practitioner of marshal arts sees her own parents murdered and her grandfather killed.  She escapes by hiding and grows up in a neighboring dojo.  When her group is hired to find a legendary city beneath the ice, she accidentally awakens a spirit who is the former emperor and he wants to rule once more.  Only he can use magic and is not hurt by the fog, though his enemies are not immune.  Our girl is the one to fulfill a prophecy, but the only way she can defeat him is to sacrifice herself beyond to wall where no one returns.