Duel

Dueling is a legal practice within the Realm, whereby two individuals may arrange a 1-on-1 engagement following certain agreed upon rules, usually for terms of honor.

In the Realm

In order to be legal, a duel must be fought in view of a minimum of 3 witnesses; one second for each combatant, and a third witness with no ties favorable to either side. Generally though, duels are fought in the open with many witnesses.
When a duel is declared, it is considered proper form for the challenged to offer the manner of combat (swords, fists, whether magic can be used, etc.) and decide the location. However, this is a formality, and both combatants must agree to the terms of the duel for it to be considered legal with the minimum number of witnesses present during the decision-making process. Part of the agreed-to terms is the duel's lethality; whether the duel will be fought to the death, to first blood, unconsciousness, etc. So long as the duel is legal, with all witnesses present, killing your opponent in the duel is fully legal under Realm law.
The job of the witnesses is to ensure that the duel is upheld to its agreed-upon terms. Each second is responsible for the concern of their party, such as getting them to a doctor in cases of injury or preparing their body in case of death. If their party does not show for the duel, or is unable participate for one reason for another, the second is expected to participate in their place. If they do not, the duel is considered a loss for the party in question.

In Arkhosia

In Arkhosia, honor is considered much more sacrosanct than in the Realm. Duels must be fought in plain view; there is no required minimum number of witnesses, just that the duel must take place in a public space. All duels are fought to the death; to leave an opponent alive is considered highly insulting, and the losing party must commit ritual suicide or else be dishonored for life. No combatant declares a second, and failing to participate carries extreme dishonor as well. When a duel is declared between dragonborn, and accepted, one combatant must die.
It is generally considered dishonorable to decline a duel, but in practice frivolous challenges are frowned upon, and an individual who declines one would not be found at fault.