Heroes in Heroes of Might and Magic V: graphics, biographies, skills...

Human Necromancers can only become immortal if:
they become Liches
they become vampires
Otherwise, it's impossible.

I don't argue. But, as is known from numerous films, the appearance of vampires is not entirely new, and the overwhelming part of a vampire's appearance is that of a former human. The form of existence will certainly be completely different - distinct from a simple mortal.