This results in "The Necromancer's Adventure."

I don't quite understand how Varens can be associated with a necromancer (the term means "one who is fascinated by the dead")? Meanwhile, of all the "darkness," only "vampirism" has an allusion (i.e., a hint) to necromancy. The main lever for controlling combat in "Heroes 5" is changing the initiative of creatures. A general speed boost can be obtained even without "light" – "speed magic," but a general "slowdown" can only be taken from "darkness." If there were a perk like "joy of braking," then it would be a completely different story.

But if there are monsters among the enemies that are not susceptible to this magic, it is useless.

There are no monsters on which dark magic has no effect. There are those that are immune to some of its spells. For example, the Black Dragon – spells do not affect it, but you can apply "vampirism" to your squad. And "darkness" will not be useless.
What to choose from the schools of magic is a matter of preference; tastes differ.