Counter to darkness spell?

Solar Brillance blinds everybody & addtionally blasts undead, check the manual for the specific details. You can forge the forbidden light for the same effect, but it's not going to win you the battle on its own.

ME Ermor mages are phisically weak, so magic assasination & BF wide spells as others have mentioned might work. Killing the caster does the job, but so does killing the communicants before Darkness is cast.

For combat while the bf magic does its job: use SCs fitted out with high protection (because of the penalty to defense) & aura/shield damage: fire shield/charcoal works best vs undead, cold/poison fumes might also (most undead are immune to this, but not vestals IIRC).