Necropolis versus Orcs.

That's the problem – there's nothing to counter the Darkness spell with Orcs. The Silence spell is too weak.

With Goblins, of which there will be thousands, the chance of dispelling the spell will be high. On a large map, there's a good chance of finding the Gnomish Shield against Berserk, as well as the Gnomish Helmet against Blind, although it can be removed by Chieftains or the War Cry skill. As for controlling units, a well-timed Darkness spell, even on a Cyclops, will provide protection. There's also the option of feigning control of the controlled unit and preventing it from attacking its own. As a last resort, take a couple of talents or artifacts for resistance. If you consider everything, it's a decent counter to Darkness. But the Necropolis army falls apart against an Orcish army under a good Blood Wrath.
However, if the battle is in the early stages and the Necromancer has Darkness, then it's already bad.