FYI, we have ruled in our sagas that "magic" was a major focus (as was trees for herbam), and "transform, destroy, control, create, investigate magic" minor ones.
Magic in the sense of "spell with magical realm source", thus excluding items, faerie/infernal/divine powers.
Useful focus for your vim specialist if he wants to... but no more than the "necromancy" for your necromancer.
In a subsequent saga, we, helas, ruled that focuses were overpowered (characters with one and characters without were not equal in power or even fun in the eyes of the ASG...) and banned them, to my displeasure, but currently I play with it and "why not".