In many ways, this is a flagship example of a typical Icelandic Saga.
Here we have a narrative of a multi-generational feud, and how two families came to blows over a minor incident. The violence escalated as each faction vowed to avenge ever-increasing assaults upon their relatives, property, and family prestige.
At the center of it all, is the ill-fated love between Kjartan and Gudrun. Kjartan, a powerful up-and-coming warrior in his youth, sailed off to Norway to seek his fortune in the service of the king. Gudrun, the most glamorous woman in northwestern Iceland, waited some time for Kjartan to return, ...but married Kjartan's best friend Bolli Bollason instead.
Kjartan eventually returned to Iceland, wealthier and with a great reputation earned abroad. Finding Gudrun married, he chose another woman, ...and Gudrun never forgave him!
With the inevitable tensions set, it was all a matter of time before the swords, knives, and axes came out.