Garzvhog was born in a ramshackle village in the Spine. He was a veteran of many battles. He later rose to the highest rank in his tribe, Bolvek Tribe, and gained the title Nar (which is the urgal word for chief). He earned this title by killing a gigantic cave bear with his bare hands during his first hunt with his horns.
Battle of Farthen Dûr
Garzhvog, like most of the Urgals, was bound by Durza's spell and forced to fight for the Empire. It is possible that Garzhvog was at the Battle under Farthen Dûr. It is also possible that he was the Urgal chief mentioned by Murtagh but unlikely as it states in Eragon that Eragon stabbed the chieftain who pursued them through the chest . When Eragon killed Durza, the Urgals were freed from the spell.
Alliance with the Varden
Just prior to the Battle of The Burning Plains, Garzhvog met with Varden leaders. He wanted to get revenge on the Empire for the wrong that had been done to his people. Although Eragon and Orik were suspicious of the Kull, Nasuada accepted Garzhvog's proposal.
Garzhvog and three other Kull served as Eragon's bodyguards during the battle, thus earning the Dragon Rider's respect.
Later, when Eragon went to Tronjheim to assist in the selection of a new dwarf king, Garzhvog accompanied him part of the way there.
Garzhvog realizes how precarious his treaty with the Varden is. He knows his people are doomed either way, if Galbatorix wins or looses. The Urgals seem to be acting out of their current hate towards Durza's magics in Farthen Dûr.
Behind the scenes
When the Kull allied themselves with the Varden, it was seen as one of the biggest "shockers" of the series. It had been foreshadowed, however, during the philosophical conversations between Eragon and his tutor, Oromis, as they discussed whether or not the Urgals were inherently evil.