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He was once a sergeant-at-arms in the service of King Brandel, but went on to become a farmer. One year Gareth was unable to pay the increased taxes that the OrlesianKing Meghren had imposed on the populace and because of that an Orlesian commander raped and murdered his wife. Afterwards, he killed the commander but he and his son had to abandon their home and live in the wilds. Eventually, he went on to lead a band of rebels in the Southron Hills with Sister Ailis. After the death of Moira the Rebel Queen, her son, Maric, stumbled upon Gareth's group as he fled her killers who took care of him and treated his wounds without knowing Maric's real identity. When Orlesian forces surrounded Gareth's camp and Maric revealed his identity, he ordered Loghain to escort the King out of the camp while with some of his men he distracted the encroaching forces allowing them to escape. For his sacrifice, Maric knighted him before they escaped.
He does not share his son's surname as it was given to Loghain after Gareth had already died. Gareth's family is said to come from the region of Ferelden known as Oswin -- if there were two Gareths present, the man would introduce himself as the one from Oswin.