According to the sagas, Harald Fairhair (Finehair in this show) swore not to cut or comb his hair until he had unified Norway into one kingdom, and married the woman he first made that vow to.
Halfdan the Black (Old Norse: Halfdanr Svarti) was a ninth-century king of Vestfold, which now comprises current capital Oslo and surrounding areas. He was the father of Harald Fairhair, the first king of a unified Norway. In this show the two are brothers. His name was given to him because of his black hair, which was (and is) very uncommon for Norwegians.
One of the few episodes with a pre-title scene.
The shot a face half-submerged is recycled from an earlier episode: the Profit and the Loss in season 4.