The ability to embody the characteristics and powers of the savage barbarian archetype. Note: that this is essentially a definition of an archetype/concept.
The user has gained insurmountable bravery and an almost inhuman set of abilities derived from rites of passage and special training received from generations of barbarians that came before. Barbarians live by a primitive code of conduct, which manifests itself as a natural will to fight for a cause or an instinct to both protect family and community to destroying the enemies that would threaten them. They, however, are not fighting for a greater-than-life ideal: To follow the path of a savage warrior is to neglect certain aspects of one's human nature, such as suppressing empathy and committing acts of extreme violence, such as pillaging the home of a defeated enemy, torture, humiliation and assassinating individuals that have little to no doing to the conflict at hand (women, children, elders).
Barbarians are archetypically portrayed as a being who can take heavy damage and use the heaviest weaponry available (although many "barbaric" cultures use little to no cloth in battle). Another staple is the presence of the berserker state, a mindless rage that overtakes the warrior during and after a ritual. This frenzy is the barbarian's greatest strength and greatest weakness, due to not only numbing their body to pain and dramaticly increasing strength, but also blinding their own sense of judgement. The savagery inherent to them is not a stereotype however. Instead, it is a mark of their philosophical way of thinking, the belief that one must embrace the deepest aspects of their nature be it for good or for evil, always handing their life to their primal instincts and being guided by them. "what doesn't kills you makes you stronger" is probably the best definition of their belief.
The Saiyans (Dragon Ball Z) are a naturally aggressive race. Even infants have a tendency to be violent.
Barbarian (Valkyrie Crusade)
The Kaguya Clan (Naruto) were known for their savagery and lust for battle. They held no plans when going into battle other than to show off their power and slaughter for fun.
It was this reckless attempt to prove themselves that lead to the Kaguya being wiped out. Even when overpowered, the clan chose to fight to the death simply to satisfy their bloodlust.
Kaido (One Piece) is a merciless warrior and one of the strongest pirates in the world. His crave for the excitement of bloodshed has given him the desire to start the biggest war in history. Kaido also doesn't take well to excuses and won't hesitate to kill those who don't give him what he wants, not even his own crew.
Much like his captain, Jack (One Piece) displays a barbaric and irrational personality. His wrathful nature goes as far as attacking children and even hacking off limbs to get what he wants. Even if Jack is given the option to negotiate, he’ll bluntly refuse.
Charlotte Linlin (One Piece) loves sweets more than anything else and will destroy entire countries over the inability to provide them. Linlin believes no pirate should compromise on what they want and therefore is very difficult to reason with.
Eustass Kid (One Piece) is a notorious pirate known for causing numerous civilian casualties. If someone should so much as look at Kid, he will end up brutally attacking them.
Hody Jones' (One Piece) hatred for humans makes him even more dangerous than Arlong. He'll not only kill humans, but even his fellow fishmen who sympathize with them.
Yama's (One Piece) barbaric behavior makes him show little to no respect for the world around him. In order to wipe out a single enemy, he will run wild, smashing any of the precious artifacts and ruins in his way.