A holmgang is a fight between two individuals, similar to what we know of as a duel. This was recognized by the society as a legitimate way to resolve a dispute.
The Norse thought of honor and reputation as being very important traits, so if either was challenged then the offended person could call for a holmgang. In early times, even a simple insult could result in a call for holmgang. Someone might also issue such a challenge if they were accusing someone of theft or had some other legal dispute.
The actual holmgang would happen 3-7 days after the challenge was issued. If the person being challenged/accused didn't show up then they would be judged as guilty or wrong. If the person making the challenge didn't show up then they would be judged by the community as níðingr and possibly even made outlaw.
During the time between the challenge and the battle, the parties may discuss the rules, including any results. Their negotiations must be agreed upon by both sides or the standard rules will apply. Either party may designate a second to fight in their place, without needing it to be accepted by the other side.
First the agreed upon rules are recited. If no rules have been negotiated, then the standard rules will apply.
The fight itself takes place in a small enclosure. Often this would be a small island or a fenced in area of about 10 foot square.
Shields have 4d4 (9) hit points each. When an attack roll results in a hit against someone wielding a shield, it is assumed to be a hit on the shield unless it is a natural 20.
If one of the combatants dies, then the victor inherits all the loser's property. This is not considered a murder, so the victor cannot be challenged for the loser's death.