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Aura of Fear + Vampire Touch – turns any warrior into a vampire, as good as the basic vampires.
Misfortune + Suffocate – possibly the only damage spell that benefits from luck.
On Bone Dragons or a Witch Hunter hero, First Strike or Serpent Strike + Sorrow on a living opponent – any living opponent without First Strike and magic resistance will be killed in any quantity with the right battle tactics.
Feline Agility + Accuracy on Medusas – few will survive a ranged attack.
If I remember anything else, I’ll write it down.