From publisher blurb:
Destroy your enemies with the power of black magic!
With the new black mage class for Adventurer Conqueror King and Labyrinth Lord, you can add arcane destroyers to about any OSR game. Unlike their fellow magic-users, they have mastered the art of elemental attack spells, even learning to use some as innate powers instead of simply spells. Originally appearing on my old blog, this class has been updated and expanded to include conversion details for Labyrinth Lord.
In addition, there are three new spells for Adventurer Conqueror King, and seven new spells for Labyrinth Lord (though obviously there can be some cross over there as well!)