Gus hails from a Dwarven mountain clan. They worshipped the Gods of the Mountains (Gus always snarls when he says this) and feared all things magical, living simple lives and prioritizing physical combat above all else.
Gus was a promising young member of his tribe's most elite warrior unit. He fought in skirmishes against a nearby underdark village of Drow, and ultimately led the charge to wipe the village out.
The village consisted of many refugees from a Drowic civil war; they wanted no interaction with the outside world and tried to explain this to the tribe. They went unheeded; the tribe feared their magical ways and their reputation as Drows.
During the slaughter, Gus found a magic relic held in honor in the Drow village. To prove his worth as a warrior, he raised his axe high and smashed the relic. This unleashed an arcane explosion that scarred and burned him, rendering him unconscious for weeks.
When Gus came to, he felt different. The first accident happened during a dinner in his honor. As he raised his horn of beer, surrounded by praise for his victory and survival, he felt energy suddenly surging forth from deep inside him. A firey blast erupted from his fist, horribly burning everyone at the feast.
Gus was deemed tainted at best and a traitor at worst. He was exiled for life.
Gus fell into a deep depression, wandering through the wilds until taking up residency in a small town at a crossroads. His physical prowess led him to a successful run as a prize fighter, and he made a few friends of different races and backgrounds in the village. He learned more about magic, and realized with horror that he had participated in a pointless slaughter. The Drow had committed no crime but be magic users.
Between his exile and deep wells of guilt, Gus became addicted to drink. It began affecting his fighting. He realized he could tap into the magic inside him to get his edge back. He reclaimed his prize fighting status, training during the day to control his magic and use it to subtly trip up his foes.
Eventually, Gus was found out. He was run out of the village. At his lowest--addicted to ale, homeless, friendless--he had an epiphany. Instead of thinking of his magic as a curse, why not use it as a tool for good? He had slaughtered the innocent village and for that there'd be no true forgiveness, but he could at least perform a life of penance in the hopes of making up for it.
Now, Gus seeks out adventure as a chance to do good. He wants, single-mindedly, to do the most good for the most people and root out evil and ignorance wherever he finds it. Unfortunately, he's still a drunk, but his fists and wild magic can get him out of most sticky spots (even if he can't remember it the next morning).