Well, I GMed a duel last month. It was a librarian against a tech Marine, and they used it to settle a dispute. I figured with all the different, highly weaponized cultures coming to a crucible in the Deathwatch, DW is bound to have rules for that.
- Interestingly, in a sparr or non-lethal duel, you have the reverse situation from actual combat. You do not want to kill or maim your opponent, which especially in the DW would be frowned upon as this could create some bitter resentments from future guys to come to the DW.
So, fighting someone from another chapter probably is a lot more ritualized than fighting within your chapter. I figured there's a duel ring -hall somewhere. Then, the duelling must be pretty ritualized and permission to participate is required at least from your captain, more likely from the watch commander. In the long months of transfer to missions or hangin' out on the fortress, the natural aggression of these guys, player or otherwise, must be reigned. So I went with a duel ring, using only the Astartes Dagger and both combatants wearing only a tunic. Within the duel ring, only certain moves are allowed (essentially the Lore Skill gotten at creation teaches the characters which one) and per game effect that means you can not use any real maneuvers that require any sort of talent or skill. Basically, the ritual reduces both characters to their basic WS and S with an Astartes Knife and nothing more. No psi, no cyber, no four attacks.
Here comes the tricky bit - if you roll a "10", then this will be automatically a confirmed righteous fury unless you successfully roll WS. Seriously maiming a fellow space marine is followed by penitence and killing a fellow brother will result in shameful discharge from the Deathwatch. Fate can be used to pull your swings.
(So yes, fighting in the Duel ring is more dangerous than without, but levelled).
Brothers seeking permission to fight in the Ring more than once or twice during their stay with the Deathwatch are ordered to sessions with the Chaplains - after all, the true enemies of the Empire do not lie in waiting with the Deathwatch.
So, in summary
- can only use dagger and tunic
- no talents, no psi, no cyber - only basic S and WS
- requires Captains permission
- automatic confirmed righteous fury
- critical hit results in Reputation loss (D10) and pennance, killing a fellow space Marine in expulsion
- Can only duel once or twice per tour of duty (alternative: once per level).
- Can not disengage (small circle)
- Hitting a brother that has yielded results in Reputation loss and pennance.
All the best, hope you like some of these ideas,