In the end, my fire mage had 26 points in fire magic and +12 from the elemental staff = 38
it turns out that the Lich's Crown adds to dark magic, but I didn't pay attention.
Now I've checked: leveling up the skill to 19 adds +9, for a total of 28 - you can deal some serious damage :D
Basically, the crown is the same as the staff, but for dark magic; the bonuses are equivalent. The rings have the exact same effect. It's a pity that the effect from two items doesn't stack, which is the case with other skills as well.
