Dominions 3: The Awakening/Spells & Rituals/Unholy Protection

There are several "levels" to this spell. The lowest (1H) adds magic resistance to one undead unit. The next highest (2H) adds magic resistance to all undead units in a 10-square area, and the highest (Protection of the Sepulcher, 3H) is battlefield-wide and affects all friendly undead units.

Regardless of the level, the precision on the spell is 100 and it costs no fatigue to cast, making the line of spells quite attractive. The spell adds four magic resistance to an undead unit (so a longdead will be boosted to 14, a longdead horseman to 14 and a behemoth to 17); this makes the unit less likely to be banished by the enemy priests.