Soul-Fire is a liquid that comes from soul-fire streams. It is a chrome colored vicious substance that has all the properties of positive energy. The blacksmiths of Holy Sanctum were the first to put the stuff to use on a large scale. During the Black Tide War, tens of thousands of arrow and bolt heads were mixed in forges with other metals creating soul-fire metals.
The Zagul-Tîn know of a location of a soul-fire stream in one of Hlothruk's valleys. In the Athenian Crusade (1539 - 1551), the werebear army fielded by the Zagul-Tîn proved particularly useful in fighting the undead. This was because the werebears ritually anointed their claws in liquid soul-fire, which is devastatingly effective versus undead.
Soul-fire weapons, both natural and manufactured score an additional 1-4hps of damage per die cast when used against undead.