fantasy worlds dont neccesarily have magic.
That does not mean that all fantasy words need not have magic. The only issue I see with this, given the presence of necromancy, is probably just balance. Sadly, that's the biggest issue possible for a suggestion.
The only ways I can see to balance something as incredibly powerful as magic is one or more of the following:
1) Make it so that magic isn't incredibly powerful, but that would kind of defeat the point.
2a) Make it so that magic is incredibly costly to the caster, costing blood to cast for example, too much casting would kill the caster (perhaps resulting in a unique state, spellhusk perhaps?).
2b) Make it so that magic is just costly, requiring certain items, plants, wands, stones, runes, ETC before you are able to cast, meaning that a powerful wizard would end up carrying around a lot of stuff.
3) Make it so that magic is not guaranteed to work, or not guaranteed to work right... Spell failure chances aren't too hard to do.
Personally I prefer having a chance that not only will a spell fail but can fail disastrously: spells affecting the wrong target or even harming the caster, summoning enemies instead of allies, undead monsters going out of control if you have too many... That sort of thing (obviously for fortress mode, a caster would have to be on player command only, so you don't have a wizard trying out their summoning skills and filling your fortress with armok knows what).
It is possible to balance magic, depending on how powerful it is.