From A Wiki of Ice and Fire
The Thenn consider themselves the last of the First Men. They speak the Old Tongue and most have only a few words of the Common Tongue. They are led by the Magnar, who is considered more a god than a man by his people. Because they have laws and lords in their valley, some consider the Thenns more sophisticated than other free folk and closer to the people south of the Wall. They have more dealings with giants than other men.
The Thenns are savage fighters, but because of their belief in the Magnar as their god they are absolutely obedient and more disciplined than other free folk. They are often better equipped than most free folk, with bronze helms, axes of bronze and a few of chipped stone, short stabbing spears with leaf-shaped heads, shirts sewn with bronze discs, and plain unadorned shields of black boiled leather with bronze rims and bosses.
A Storm of Swords
The Thenns are led by Styr, the Magnar of Thenn, who rules with an iron hand. The Thenns have joined the army of Mance Rayder, who defeated Styr three times to their allegiance. Jon Snow accompanies one hundred Thenns who climb the Wall and descend at Greyguard, although Jon escapes from them at Queenscrown in the Gift. The raiders attack Castle Black from the south but are defeated by the Night's Watch, with Styr dying in the fighting. Stannis Baratheon captures the new Magnar when Mance's army is defeated in the Battle of Castle Black.
A Dance with Dragons
Soon after Mance is defeated at the Wall and the his host broken, a dour warrior in fur and amber urges the survivors to come with him and take refuge in the valley of the Thenns. Though the Thenns themselves had fled their valley to find safety in the first place, hundreds of free folk still follow the warrior.
|“||The Thenns have lords and laws ... They mine tin and copper for bronze, forge their own arms and armor instead of stealing it. A proud folk, and brave.||”|