If you look at the picture of the dragon's soul guardian that was posted earlier, it actually has both a black and a white magic book chained to it, like those on the dark and light pact skills, so it is probably the same one for priests and dark lords (and wizards, presumably).
Lv. 22x SM; lv. 25x (mad) Scientist
Amatsubame, Lv. 68 Chaosmage
Lv. 85 Elf Loremaster