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Skeletons - there are many of them +1
Ghosts - drain mana, regenerate +1
Vampire Lords - drain life +2
Liches - cloud of death that doesn't attack allies +2
Death Knights - double damage +1
Dragons - decrease enemy luck +2
Veil of Darkness +2
Guild of Mages level 5 +3
Spellcasting +5
Chance to hit in the Guild of Mages, raising the dead 90% +3
Grail - increases black magic by 20% +3
Independent of morale +2
Total: 27 (but it's unlikely that these are all their advantages)