Final answer:
Brutal Critical is a Dungeons & Dragons barbarian ability that allows extra damage dice on critical hits, beginning at 9th level and increasing at higher levels.
Step-by-step explanation:
Brutal Critical is a feature that grants a barbarian in the game Dungeons & Dragons the ability to roll additional damage dice when they score a critical hit. This feature becomes available to barbarians starting at 9th level. When they land a critical hit with a melee weapon attack, they can roll one additional weapon damage die to add to the extra damage of the critical hit. As the barbarian reaches higher levels, they can roll more additional dice: two additional dice at 13th level and three additional dice at 17th level. This represents the barbarian's ability to strike with ferocious power.