There are creatures so large and horrifying, or supernatural horrors so unholy and terrible, that their mere appearance on the battlefield can cause the bravest and most steadfast of veterans to turn tail and flee.
Source: Warhammer Fantasy: 8th Edition
Models that cause Terror also cause, as discussed earlier in the chapter. In addition, a model that causes Terror is so, well, terrifying, that other rules also apply. It should be noted at this point that Fear-causing models, being quite scary themselves, treat Terror-causing monsters as causing Fear, rather than Terror – this is an exception to the rule that makes Fear-causing creatures immune to Fear. Terror-causing models are themselves immune to both Fear and Terror. This includes characters riding Terror-causing mounts, which count as causing Terror themselves (see the ).
Run For Your Lives!
When a unit is charged by a Terror-causing creature, there is a chance that the warriors will abandon their position, fleeing before the creature rather than fighting it. If a Terror-causing creature declares a charge, the target unit must immediately take a panic test to quell their Terror. If the test is passed, all is well and the unit can declare charge reactions normally.
If the test is failed, the unit must make acharge reaction. Units composed entirely of Fear- or Terror-causing models are immune to Terror and so do not take this test. Note that, if the target unit is not allowed to take any charge reaction (if, for example, the Terror-causing unit charged as a result of a pursuit or a random move), then the target does not take this test.