Little is known about the Hidebarks other than they were assimilated into the Legion sometime after the First Battle of Hoover Dam, and that the Hidebarks are Legate Lanius' tribe.
Before the assimilation, the Hidebarks were very hostile to the Legion and had staged many ambushes and raids on Legion troops. Sources are unclear on how long it took for Caesar to conquer the Hidebarks, ranging from years to only several months.
Either way, the tribes conquering culminated when their chieftain decided to surrender, seeking to spare most of the tribe from pointless bloodshed; Lanius did not take the news of surrender well and killed the chieftain and fifteen of his tribesmen before being knocked unconscious. The men and boys who were not killed by Lanius were forced to become Legionaries under punishment of death, while the women and girls were forced into slavery. Prior to Lanius' agreement with Caesar, he was allowed to go forth and kill all the surviving adult males of this tribe.
↑The Courier: "What can you tell me about Legate Lanius?" Antony: "It took years for Caesar to conquer Lanius' tribe, and Caesar showed mercy to the survivors, allowing them to join the Legion. Had the positions been reversed, Caesar would be dead. Lanius is called the Monster of the East because he never, ever shows mercy." (Antony's dialogue)
↑ 2.02.1The Courier: "I've heard rumors of your legate, Lanius. What's his story?" Caesar: "Lanius is the greatest of my battlefield commanders. Some might call him a great man, but I'm not sure he qualifies. Once, he was the greatest warrior of the Hidebarks, a tribe of the Arizona. Maniacal in battle. Sometimes he'd ambush Legion patrols by himself. When, after several months we found and surrounded the Hidebarks' camp, their Chieftain raised a banner of surrender. The warrior who was not yet Lanius went insane with rage. He struck down his Chieftain and attacked his own tribe." The Courier: "What happened when Lanius attacked his own tribe?" Caesar: "He killed fifteen before they brought him down. He didn't die, obviously. I had him tended to. He was maimed, most of face torn off. It was days before he regained consciousness. When he did, I went to his bedside and showed him the helmet I'd had forged to cover his face. I said he could have it if he'd fight for me. He accepted... on condition that he be allowed to kill the surviving males of his tribe. I said, make it the adult males and you have a deal." (Caesar dialogue)