Heroes 5.5 WarGame Edition - Tactical Edition.

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

That's exactly what I'm talking about! - this is what primarily defines "vampirism" - not draining, but becoming undead, which in turn is not related to darkness, but to necromancy (that is, in my opinion, vampirism doesn't belong in darkness)

Added 2 minutes later
Oh yes, I forgot - I'm talking about the lore and the structure of Ashan, and this mod doesn't particularly rely on the lore.