Short version: The Jómsvikings are the Avengers of the 10th century.
More accurately: The Avengers are the Jómsvikings of the 21st century.
The Jómsvíkinga Saga is an Icelandic saga written sometime in the 13th century. It takes place in Denmark, Norway, Wales, and Jómsborg, and ends with the Battle of Hjörungavárg in 986 AD. It details the (long and complex) family background, deeds, and eventual downfall of a group of select Viking heroes known as the Jómsvikings. Much like the Fianna of Ireland, they are a brotherhood of elite warriors, the best of the best, not admitted based on anything else but bravery and personal prowess. Both of which there is a lot to go around in this story.
The heroes of the saga are the Jómsvikings, living in their own walled city (Jómsborg) under strict rules - men only, loyalty to each other, no preferential treatment, equal distribution of booty, that kinda stuff. The founder of the place is Pálna-Tóki, and after his death Sigvaldi, who is not as good a leader, and manages to let the whole thing crumble on a drunken bet (great job, Sigvaldi).
One of the most notable characters is Vagn, who shows up at the water-gates of Jómsborg at the ripe old age of twelve, demanding entry into the brotherhood. He is refused on grounds of creeping everyone out, and also the fact that he is underage (rules only allow membership to men between eighteen and fifty). He proceeds to soundly beat Sigvaldi in a naval battle, and gains admission anyway.
Apart from your usual run-of-the-mill fighting and pillaging, the saga offers a few remarkable moments, especially towards the end. They mostly involve fighting:
1. Much of the end of the saga (I'm not giving out spoiler alerts here, the story has been out since the 13th century, deal with it) involves the captured Jómsvikings being executed by their enemies. It is a fascinating lineup of warriors going to their death, each one making a different joke or proposing fun little experiments. One of them insists that his long and fabulous hair has to remain clean and intact; then, just when the executioner swings the sword, he yanks his head down, and the sword takes off the arms of the man who had been holding the hair out of the way. The kid (barely 18) looks up and asks with a shit-eating grin: "Whose hands are in my hair...?" He gets to live, Vikings appreciate a good practical joke.
2. Another battle-related moment of epic is delivered by Búi, one of the Jómsvikings, who loses both his hands in the last battle of the company. Instead of being fazed by this at all, he puts the arm-stumps through the hoops of a chest of gold, and jumps overboard with it, transforming into a dragon.Yup.
3. And still on the topic of lost appendages - I'm sensing a theme here, you guys - there is some bravery on the opposite side as well: Geirmundr, the commander of a town attacked by the Jómsvikings, jumps out of a building, gets an arm cut off by Vagn, and then manages to run through the woods all the way to the earl's court to warn him of the impending danger. He does it in a calm, collected and professional manner, reporting on numbers first, before the earl asks him if he has any proof. To which Geirmundr responds, "Oh yeah, they cut off my arm, see?" and follows up by providing information on the enemy's best warriors. "One of them got my ring."
Ain't no party like a Jómsborg party.
The Real Jómsborg
Although contested by some historians and archaeologist, it is generally believed that Jómsborg stood somewhere in the place of modern day Wolin, Poland. Every year the town celebrates this fact with Europe's largest Viking festival, involving historical reenactors from all corners of the continent and beyond. Definitely worth a visit.