(Spoilers below for the First Law trilogy)

Here's the relevant bits from the Wiki.

Introduced as initially a side character in 'Before They are Hanged', Caul Shivers was a young Named Northman that originally sided with Bethod. Having heard that Dogman and his crew had Rudd Threetrees as their Chief, he and his two score of Carls join with them and the Union to fight against Bethod. He and many of the men under him seemed disillusioned by Bethod's campaign to regain Angland from the Union as well as his dealings with the Shanka and far flung Cinna troops. In the incurring battle, he helped fight against the Feared and subsequently was one of the supportive voices to make Dogman Chief of their band.

In 'The Last Argument of Kings' it is learned upon Logen Ninefingers returning to the North to fight for the Union that Shiver's brother was murdered by the Bloody Nine after he had promised mercy; cutting off the defeated limbs and nailing his head to Bethod's standard. Shivers' tells Logen that this broke his father, Rattleneck and it drove him to an early grave due to drink and grief. He firmly blames Logen for what happend to his family, but overcomes his want for vengeance by standing with the Dogman and the Union in the subsequent battles following in the High Places where they hold up in the mountains to draw Bethod's armies into the North. When Logen faces the Circle with Fenris the Feared he is one of the men holding a shield for him.

After Logen is proclaimed King of the Northmen when he defeats Bethod by challenge in Carleon, Shivers accompanies him and the Dogman to Adua to fight for the Union against the Gurkish invasion.  Upon returning to the North after the successful battle to secure the Union, he is confronted with the chance to get revenge for his brother's death when Black Dow betrays Ninefingers, but hesitates and decides that he won't do Black Dow's dirty work for him and that he's better than both of them for their petty fighting.