Light for the Necromancer.

I still don't understand... should I take it just because it's incredibly cool and rare?
What's the point of taking Leadership if it doesn't give Battle Discipline? But if it does appear, then Diplomacy, Messenger, and Money are good choices.

And Light would also be good if there are spells. It has Righteous Strike and Stone Skin... and so on.

No, 3 Magic won't work; there are more useful skills for a Necromancer, such as Education. Righteous Strike and Stone Skin won't help much because the damage and defense of the undead are too low. If you were to develop attack and defense, then Stone Skin and Righteous Strike would be useful, but 100% Magic is useless for a Necromancer; it's more for a Warrior.

P.S. although Education is not among the skills listed above, so it won't be possible to take Empathy.

So, Empathy doesn't work for the undead because they don't have morale, so the hero won't be affected, and Money, Diplomacy, and Messenger aren't worth sacrificing a skill for