In the big city of Everythingville, there is a particularly quirky fighting academy.
Eschewing the training of moves and patterns that the better fighter schools espouse, these creatures seem more closely allied with the Ranger school, only slightly less understood.
Though the school values wilderness training, but the emphasis is more on enduring harships and withstanding blows. Armor training isn't part of the package, but fighting skill is still honed. But there is also an emphasis on coping with the wilderness.
While the Ranger focuses on hunting and mobility, the Barbarian School focuses more on embodying aspects of nature in an almost shamanistic sense. They consider nature a wild, untamed essense, and this is emphasised in their training of their warriors to go into wild, untrained spurts of passion where they tap their inner beast, storm, or whatever. This takes a certain amount of training, as the school argues that it isn't an entirely instinctive, since the life of a typical humanoid squashes this side.
Thus, the warriors of the Barbarian school are trained to tap their inner nature. This kind of training is so contrary to much of their nature, that graduates from this school often have trouble in reading and writing the scripts they may have been familiar with beforehand, considering language too much of an 'artificial construct of a society built on the fallacy of order and patterns.' Some train themselves to read and write anyway, but it is difficult for them, in general.
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Bingo. Someone becoming a barbarian. No more extreme than otherwise.
In a more typical campaign, the character probably spends the level trying to loose control, trying to give into the rage inside of them, trying to become more instinctive in nature. Anybody's capable of gaining the abilities.
Of course, I belong to the 'classes are only packages of abilities' school of thought, so this is easy to reconcile with me.
