Rune of Tallon
Rune of Tallon is a clicky item that comes from Plane of War, a zone added during the Call of the Forsaken expansion pack. In order to get to the side of the zone that The Barb is on in Plane of War you will first need to complete a quest to get flagged for the Field of Strife. For more information about how to complete the quest, head on over to my Field of Strife Flagging Quest Guide.
Once you're able to enter the Field of Strife you'll need to defeat about 30 enemies to summon The Judicator - a named that is really difficult for its level - it'll likely still be a challenge to you even when it cons light blue or green! Defeat him and loot the Warlord's Insignia which is what will give you access to one of the two castles on the eastern and western ends of the Field of Strife.
Finally, you'll find The Barb in Castle Rulnavis in the western portion of the zone. As for how to beat The Barb, well, he's a bit of an annoying named but also extremely easy. He moves extremely slow and he pushes/pulls you and your group around the entire area. You will need to clear out the majority of the trash mobs in the room before engaging him and you should pop your cooldowns immediately after pulling him so that you can quickly dispatch any adds that come with him as well as take out The Barb as quickly as possible.
His only special ability is his pull/push and his HP locks. Occasionally throughout the fight The Barb will go invulnerable and I nor anyone else I know has ever figured out how to make him drop his Invulnerability. I've tried running him around the room, not moving him - everything. Basically, the Invulnerability lasts anywhere from 10seconds to forever.... Yes - forever. Sometimes the Invulnerability will never fade from him and you have to either die or evac your group and try the fight again.