Brothers in Arms is a quest received from the goblin brothers Nathirap and Eareb in Aleroth. The two goblins are initially naked and standing outside of Willy's House. They tell you how they want to fight for Aleroth, but are not allowed, and ask you to appeal to Aquila, the Champion General, on their behalf.
Speak with Aquila, who is located just west of the goblins. We will tell you he does not believe the goblins are true warriors due to their lack of gear.
Return to the goblins to give them the news. You will need to go speak to Kaan, the goblin merchant, who will now have a new dialog for you. He offers to sell you Goblin War Equipment (quest item) for 11,000 gold. The equipment is "a full weapon and armour set for goblins, made in the Red Hammer Tribe village in the Orobas Fjords. This is probably the last armour set ever produced by their smith."
If you go through with it, you are able to return to the goblins, who will then put on the gear and lead you to Aquila. Speak with him again to receive your reward.