They are usually furious about being forced into temporary servitude. I like that! They have no choice but to take it out on my enemy.
Summon Devil is a fifth-level Demonology spell in Heroes of Might and Magic IV that costs 24 spell points and summons a number of Devils based on the level of the caster. This is the most expensive spell in the game, including creature spells.
This spell summons about twice as many Devils as one would expect from any other summoning spell. While no more powerful than most level 4 units, devils combine fearsome maneuverability with mighty attack power. Frankly, this spell can produce three weeks or more worth of devils in the blink of an eye, if only for the duration of a battle. Such power lends itself to bait tactics, where your hero walks alone to tempt an AI stack into attacking, casts this spell once or twice when battle is joined, and proceeds to mop the floor with the opposition.