Don't use evil, just torture the sheep.
That won't be sufficient to do what you want.
Making a single mildly regenerating zombie cost us 1 power, and those still died when their skull was destroyed. To make a flock of regenerating sheep seems like something that would cost more power than we posses.
Anyway, let's think what we're up against.
Known enemy forces :64 soldiers
Includes : Archers
Shieldwall
77 monks
Includes : Martial arts
Anti-evil chanting
91 civilians
Known enemy fortifications - Main compound ( can be damaged by troll siege)
- Underground tunnels (include spring)
- Granary, Warehouses (accessible by tunnels)
- Outlying buildings (very vulnerable to trolls)
Ambush :
12 Soldiers vs 36 skeletons : Kill 8 soldiers before enemy escapes (dozens of skeletons lost)
4 soldiers versus 2 brutewolves : both Brutewolves killed
2 soldiers versus 6 brutewolves : both soldiers killed
Tactical Thoughts : Brutewolves appear to have trouble with armored targets, need overwhelming number
Strategic thoughts : Orders where to crush enemy with overwhelming force, not to feed our forces to them in surviveable chunks. Wights, brutewolves, midnight imps should have been present from the beginning of the fight.
Bombardement :
Rock trolls vs Settlement (11 civilians killed, many injured)
Economic thoughts : 10 physical and 2 mental, but they're reuseable
Tactical thoughts : could have gone better, I guess
Strategic thoughts : Complete failure to attain objective. Actually counterproductive, as it concentrated rather than dividing enemy forces
Thing assault
20 zombies vs 1 soldier (1 killed, many civilians dead)
20 zombies vs 8 soldier, 5 monks ( 7 soldiers, 4 monks dead , 2 zombies killed
18 zombies vs 4 squads ( 1 soldier, all zombies kilked
Economic thoughts : 10 physical, non-reuseable
Tactical thoughts : ineffective against armor, still takes casualties even if they have numerical advantage)
Strategic thoughts : Complete failure to attain objective due to being deployed outside the plan. Even so, I doubt they could have accomplished what they were supposed to do.
Zombie horde
Economic thoughts : Next time, consider priority delivery
Tactical thoughts : Great at being useless corpses
Strategic thoughts : Ensure primary army can be deployed before attacking
Shai Hulud
Shai Hulud vs lots of soldiers, priests and 24 archers ( many civilians killed, 17 soldiers, 19 monks )
Economic though : + 2 Physical , -3 Mental
Tactical thoughts : Don't attack the entire fortress at once. Retreat and attack from a different direction when facing resistance
Strategic thoughts : Only thing that actually worked
So, from this analysis it's rather clear that all the summoned units appear to have been a waste of time and resources.