Necropolis discussions.

And even royal griffons? Which will always take off before you can cast a spell...

Move your troops aside, the griffons will strike in vain, and that's when you can take them out.

Regarding resistance, most units don't have it. Besides, there are other powerful spells in the Darkness faction, such as vampirism, mass slow, and plague. Blindness also belongs to the magic of Darkness and is available to Necromancers.