Nargott
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No, if you need lore, you should see a doctor :) (I hope you understood the joke correctly).
Vampirism protects against darkness, so why shouldn't it be in darkness?
Besides, considering how strong it is (in WGE it always drains 100%), the draining effect itself is the main one.
And the very relationship between vampirism and necromancy is questionable: if vampirism is related to necromancy because of the undead effect, then why is this effect completely useless for necromancer troops?
In general, as I understand it, your position is that vampirism should be banned for everyone except necromancers, but this decision is unlikely to be acceptable to many other players in PvP, because it narrows tactical possibilities.