Fear

Fear is an effect that causes the affected creature to flee from whatever produced the effect, insofar as it is able. In Dungeon Keeper 2, when a creature is under this effect, (s)he legs it in the opposite direction, and the effect is indicated by the creature's experience/level indicator flashing with a vaguely skull-shaped icon similar to that representing the Fear Trap. In Dungeon Keeper, creatures who are afraid simply try to get away from the area via normal movement.
In Dungeon Keeper, Imps are phobic of combat against other creatures; they will tend to flee enemy creatures unless those creatures are approaching the Dungeon Heart, in which case, they overcome this fear and will fight with their lives to defend it. The Cave-In spell also produces a fear-like effect; any creatures caught in this will return to their Lair or point of origin (either their starting location in the map or the Action Point they originated from). Creatures affected by the Chicken spell will also flee until they turn back to their original form, with Keeper-owned creatures returning to that player's Hatchery, and therefore risking being mistaken for an actual Chicken and eaten. A similar effect happens to creatures when the Flee setting is turned on in the Information Panel: creatures begin to leave towards their Lair once their health gets low. Each creature has a Fear statistic, but in KeeperFX, this is split into two types: 'stronger' and 'wounded'. These are thresholds, not quantities; setting the 'stronger' value to 0 will not make the creature fearless: on the contrary, it will actually make it extremely fearful of literally everyone.
In Dungeon Keeper 2, fear can be caused by a Fear Trap, or by simply being vastly outmatched in combat. Skeletons are immune to this effect, while Mistresses and Giants are highly resistant to fear. The Thunderbolt spell can temporarily negate an enemy's fear effect by stunning them.