Hello. I am a big fan of the Necromancer faction in Heroes of Might and Magic 5 and wanted to share some ideas on how to improve this already strong Castle.
1.
Spiritual Vampirism
Required skills:
+ 
For every 3 mana units spent by an enemy hero, 1 mana unit is restored to the hero.
2. 
Undead Empowerment
Required skills:
+ 
Undead units under the hero's command gain morale. Morale cannot exceed 2.
3. 
Blood Frenzy
Required skills:
+ 
Troops under the Necromancer's command gain passive vampirism and restore 8% of their health from each attack. This effect stacks with the "Vampirism" spell.
4. Enhanced skills:
+ 
Grave Cold now returns 10% of the damage dealt to units. Cold Steel now inflicts -3% attack bonus on each undead unit attacking or being attacked.
5.
Hypnosis
Required skills:
+ 
The hero gains the ability to influence the enemy hero's spellbook, "removing" 3 random spells of levels 3, 4, or 5 from it during battle preparation (one spell per level).