Here's my Necromancer build:

Let me explain why I built it this way:
1) Dark Magic => Curse Master + Mind Master.
Dark Magic is the strongest weapon against Orcs: Berserk, Dominate, and mass debuffs will severely cripple the Orcs.
2) Enchantment: Lucretia starts with Enchantment, which is good because the more often your hero gets to act, the more nasty spells you can cast. Wisdom allows you to learn some useful spells from other schools, such as Phantom.
3) Attack: We mainly take Attack for Battle Fury; skeletons get a huge bonus to damage, plus an additional 20% from ranged attacks. Since the Necromancer's army is quite slow, we also take Haste, and +30% initiative for all units is always useful.
4) Defense is a must, because the core mechanic of playing as a Necromancer is that your troops shouldn't take more damage than they can withstand. Dodge reduces damage from Centaurs by -20%, which also contributes to survivability. And mass Stone Skin is also useful.
5) Necromancers are primarily spellcasters, so they need more spell power, knowledge, and mana; therefore, Education is essential. And the bonus to experience will allow you to level up faster.