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I have one problem that prevents me from enjoying playing as Necromancers: I don't like any of the Undead heroes.
Nadira – her special ability is practically useless in a decisive battle.
Ravenna – Chaos Magic is, of course, not bad, but still, the racial skills are Darkness and Summoning, and learning three magic schools is too much.
Zoltan – he's not effective against everyone, and blocking a 5th-level spell is not that easy. A questionable choice.
Caspar – many praise him in short games, but he's far from the best in a long-term struggle.
Orson – he might not be bad, as the zombies will live longer, which means the hero can cast more spells, but zombies remain just cannon fodder.
Deidra – I don't know how it is in version 3.1, but -1 to Luck and -1 to Morale
with the Banshee's Cry – the bonus is a laughable one, equal to the sum of the minor artifacts of the Clover and the Lion's Necklace.
Vladimir – he seems decent, but +5 to Spell Power at level 25 for raising the dead seems insufficient to me. Maybe I'm wrong here?
Lucretia – perhaps I have the fewest complaints about her. I only see one drawback – the Mana Regeneration skill. Of course, it will be very helpful at the very beginning, but it shouldn't be that relevant closer to the middle of the game.
I really hope someone can convince me otherwise; maybe then I'll be more interested in playing as Necromancers