Final answer:
At level 9, barbarians gain the abilities Brutal Critical and Relentless Rage.
Step-by-step explanation:
At level 9, barbarians gain the abilities Brutal Critical and Relentless Rage. Brutal Critical allows barbarians to roll one additional weapon damage die when they score a critical hit, increasing to two additional dice at higher levels. Relentless Rage allows barbarians to keep fighting even when reduced to 0 hit points, potentially staying conscious and continuing to attack.