Showdown in the Shadow Lands
Step 1: one character challenges another to a duel. It doesn’t have to be a formal challenge, but an understanding between the two that somethings going down.
Step 2: both characters accept and lay out conditions, if any, for the duel. This could be nothing more than a nod, or an exhaustive list of criteria and contingencies.
Step 3: An opposed Intimidate check or Deception vs Insight. Both can happen at the same time if one is going for intimidation and the other Deception. The loser has disadvantage on its attack. Winner has advantage and if they succeed it is a critical hit and does Con damage if it is a duel to the death.
Step 3: Roll Initiative if both are still standing.