Ringwraiths cause terror, if a formation charges the nazgul's formation, does the enemy f. have to take a courage test for fighting a terror causer? or it do they only take the test if they charged his company in particular? or not at all?

It was int he FAQ that if the formations are charging (whether the Ringwraith's is charged or doing hte charging) a Terror test is taken, regardless of the Ringwraith ending up in combat or not.

Seems a little silly to me, I think it should only be if the Ringwraith's company gets into combat, but rules is rules

That wouldn't work, you'd just charge another company and end up never taking a Terror test.

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"There are few left in Middle Earth like Aragorn, son of Arathorn." - Gandalf, Many Meetings

It would work, it just means that it's more likely to be a Terror test caused when you do the charging. However, that is how the rules technically were before the FAQ specified that it's one of the few abilities conferred to the whole formation by an Epic Hero.

TBH, I'd be pretty scared charging an evil spirit in a black robe that screamed like my little brother with a throat ache. Even if it was just in an orc rabble.

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"War does not determine who is right, only who is left."
- Bertrand Russel

In a world where orcs are real, I doubt I (personally) would be any more scared of a robe-wearing insubstantial than I would of a horde of orcs, lol. However, if you're fighting the orcs and not even near the ringwraith (even if he's in the formation), then why would you be scared of him?

It's charging him that's the problem. If you only had to take terror to charge his company, the sensible player would just charge another company as General Elessar said.

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"War does not determine who is right, only who is left."
- Bertrand Russel

Gamewise, I agree that it makes sense from a balancing perspective.

However, before the FAQ, it shouldn't have been played in the current way because of the wording.