Heroes 5.5 WarGame Edition - Tactical Edition.

Personally, I really dislike the "Vampirism" spell, and not because of the "Drain" ability it grants, but because it turns a unit into "Undead" – in my opinion, this feels more like necromancy than dark magic.
P.S. Question: Is the spell granted as a perk through a script, or by configuring this perk through the game's files? (I looked, but didn't find such a setting, so my conclusion is: either I didn't look in the right place, or it needs to be done through a script).

The spell is granted by a script.
Turning units into undead makes Vampirism more functional and interesting. It allows removing control, berserk, blindness, plague, confusion, and partially grief (the dark counterpart of exorcism/antimagic). It allows using Reanimation without losses after a battle.

Moreover, what is also interesting is that Blindness is also a kind of antimagic here, but if Vampirism removes darkness, then Blindness counters light. If the opponent has focused on a particular unit, applying one or more buffs to it, then Blindness will not only take that unit out of the game but will also effectively neutralize all these buffs, the duration of which may end sooner than the duration of Blindness.