Game concept
Single Combat
The game also links it as Duel, Single Combats, Duels, Sudden Death.
From the game's encyclopedia
In a Single Combat, two Characters fight, either to the Death or to first blood. Each round, both Characters select one randomly rolled option from a list, trading blows until one Character gets a solid strike in, with the options available depending on your Prowess and Traits.
Some options may give special effects, and most will increase your Likelihood of Success and Risk of Injury. As Duels go on, they grow more dangerous and uncertain, and it becomes easier to win or hurt yourself. After several rounds, Duels enter sudden death, ending immediately with the higher Prowess combatant winning.
When fighting Duels, you may gain temporary Duel Edge Bonus or Duel Edge Penalty Character Modifiers, increasing or reducing your Prowess until the Duel ends.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Single Combat is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "In a Single Combat, two Characters fight, either to the Death or to first blood." It covers Likelihood of Success, Risk of Injury, Duel Edge Bonus and Duel Edge Penalty. The game also links it as Duel, Single Combats, Duels and Sudden Death. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.