[Endoperez]

Solar Brilliance will keep blinding all units, both your and the enemy, for the whole battle. It will hurt low-unrest units, undead and demons more than others, but since demons all have high magic resistance it doesn't hurt them as much.

Have you tried raiding his provinces? He has death gems in his lab, but his gem INCOME is also important. If you take his provinces, you'll deny him the sites for at least one turn, possibly several.

Do you have any Blood magic researched? Demons don't mind the darkness, and Fiends of Darkness even get boosted.

What kind of troops is he using? Your Vanir might have high enough defense to be useful in the darkness, even with halved base defense. IIRC, bonuses to defense (mounted, mirror image, shield, experience, Water bless if you have it) are NOT halved by darkness.


EDIT: If you're going for Solar Brilliance (in Enchantment), you'll get access to few Death summons. Undead Horsemen are good for the cost, and a behemoth or two might be worth a try. Be sure to buff them any way you can - Body Ethereal would do wonders.