Gauls have some units that scare the enemy, yet the rest of their army does not really give me the creeps they seem to be somewhat the "good guys".
Lusotani ambush all the time which can make it pretty scary to pass through their territory but once you fight them they are not particularity scary in a proper barbarian manner as they either look like boy scouts or far too civilized.
Casse units on the other hand often have woad all over them thus seem a bit crazy and scary in a barbaric manner they may not have woad painted Naked dudes but afaik a lot of units that wear a shirt in gaul don't in britain mainly to show their body art. plus the presence of more chariots as chariots are scary whereas not in a barbaric manner ;) last but not least the casse have their Champions who have a sort of "scary" and "terrifying" overtone. on the other hand I always associate Casse with a lot of water and mountains which somewhat takes their "fierceness" away for me :(
Getai have the best roster of all "barbarians" that said I'd like to add that they don't have factional fear units(or do they?) which somewhat a blow. Imho they are a bit to civilised to get the "fiercest barbarians" eg. horse archers are effective and not fierce in a barbarian sort of way. they lack this crazy look in the eyes with the exception of the drapanai, <- they are fierce! when I was a soldier I would rather run away from a wild dude with a falx than a naked Frenchmen, provided off course I wear trousers and a good beltand as general they are equally scary as you know if they get to your troops they will cut them to little bits :(. another thing that makes then seem to be the second most "fierce Barbarian" is their location in a pretty dark area (in the opinion of the Coast dwelling "civilized" factions) they have their own territory behind which the total wilderness begins where the tribes are free and wild ...same also goes for the sweboz tho more extreme. which leads us to my personal "fiercest barbarians"
the sweboz: they dwell in the dark and dense woods and marches of Germania. It is not safe for a roman or Greek too travel these dark lands full of swamps, cold wind, wood demons and angry barbarians who come out of nowhere. they are without doubt the most "barbaric" "barbarian" faction after all. they are one of the few peoples who make massive use of clubs(now that's barbaric) even their levy troops are scary to a(very) small degree, Chatti youths storm viciously at thier foe with the unshaved beard obscuring their faces. their Generals fight on foot together with the other warriors they don't need horses as they don't intend to flee. the moral of the most units is better than that of the Celts which implies a certain braveness of barbarians. for truely scary units they have their (crappy) naked spearman which because of his dark body art looks much more terrifying than his celtic colegues the wargozez has to be considered somewhat a factional mercenary still i think they can go into this. in contrast to the naked fear people these guys have AP axes which is especially effective against southern invaders. as an extra one could count Basternoz falxmen and Lugian swordsmen who are armed with "scary" and"fierce" weapons as mentioned above.
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