Our name is Varaug. Fear us.— Varaug
Varaug was a shade created by three of Galbatorix's sorcerers during the battle in Feinster. The spellcasters in question were desperate to win the battle, thus they did not care whether equal casualties were sustained on both sides. Though stronger than Durza, he was slain almost immediately by Arya with the aid of Eragon.
It is likely that Varaug was more powerful than Durza due to the fact he is described as having dozens of Spirits possessing him, as opposed to Durza who is described as having only three. These multiple personalities could also explain why Varaug referred to himself in the plural sense during the battle. However this may also be because the spirits that took dominion over his faculties were not accustomed to working as a unified being.
Another explanation for why Varaug was stronger is because Durza was controlling the Urgal army, severely hampering the extent of his power and concentration. He was one of the two known shades alive during the Rider War, the other being Durza.