In my current game, I am contemplating the idea of having spars between other martials and myself during downtimes, but I haven't found much to make such spars safe. I know one can declare that they're attempting nonlethal damage, and leave it at that, but it seems like it would break immersion, so I figured I'd see if I could come up with alternatives.

My original thought was homebrewing practice versions of our weapons, but that presents the problem that lugging the practice weapons around gets cumbersome, and making and remaking new ones takes time; not a bad option, per se, but reliant on a crafting system that doesn't seem up to the task.

So, my other thought was, spells that debuff weapons. I'm currently thinking of something like the inverse of Shillelagh, where the weapon cast upon has its damage reduced instead of increased. I kind of like this chain of thought, as the more I think about it, the more spells of this nature might have actual combat applications.

I'd appreciate any feedback one would be willing to give.

Well, my first thoughts were:

1. Just declare the intention to not kill each other at the outset. That way you can beat each other unconscious without having to worry about it.
2. Instead of fighting until someone goes down, set the parameters that you’re going to fight until someone yields, and agree to a yield threshold so that the first person to hit or go below that threshold must yield, maybe 10 HP, or higher if you’re both doing more damage than that per hit.

I don't really see the problem here. Whether you want to have practice versions of weapons or a spell that turns real weapons into such, you just need to say these weapons always deal nonlethal damage. I'm not clear how you think that breaks immersion when magic can do pretty much anything in this game. Can a fighter not just use the flat of their blade or stop their swing a hair's breadth away from the neck like we have seen in countless movies?

I think your debuffed weapon ideas could work, especially if you take a leaf out of sport fencing's book. In fencing you score one point for one hit and fights are first to 5 or first to 15. You could make a blunt "sparring longsword" or "sparring staff" that deals exactly 1hp bludgeoning per hit, and have your bouts be first to 5 hits.

Following up on that, you could have a ritual that converts any weapon into its sparring equivalent for some amount of time, say an hour. Then you only need to carry one weapon; the ritual part ensures it can't be done to someone who's not willing (e.g. mid fight) as some OP debuff.