This was a discovery I made while testing out friendly fire spells on a Forbidden Fist. The monk passive ability Soul Mirror will not only reflect back missed ranged weapon attacks, but also some spells, including beam spells. When a beam gets reflected by a monk this way, it creates an entirely new, separate beam that originates from the monk and bounces back at the caster without further harming the monk. This new beam also uses the stats of the original caster to calculate damage, accuracy, and penetration, though visually it looks like the monk is the one firing it back at the original caster. Additionally, the monk doesn't need to be the initial target of the beam for this to work. They can walk into the path of a beam, get missed and start reflecting it, then walk out of the path to safety. Here is a screenshot if this is not clear:
In this example, I used my wizard to cast Ray of Fire on the Rotghast. I then walked both of my monks into the path of the spell, waited until it missed and reflected new beams, and then walked the monks out of the way so they wouldn't get hurt any more. So the middle beam is the original hitting the Rotghast, while the two new branching beams are reflecting from the monks back at the wizard. Why is this noteworthy and how can it be exploited?
The obvious choice is for the wizard to use the Ray of Fire beam and wear Rekvu's Scorching Cloak while suffering from an injury (Serious Burn giving the most synergy here) so that they are immune to fire damage and can convert 10 percent of that damage into healing. With high might and a couple of spell damage modifying items, I was able to deal an average of 20-30 fire damage, translating to 2-3 health gained per tick. While this may not seem like a lot, compare that to Wall of Flame, another fire spell that I'm sure others have tried to use with Rekvu's Scorching Cloak for healing purposes. The problem with using Wall of Flame this way is its base damage is simply too low, so you'll often see it giving 0 health back when it ticks. Meanwhile, Ray of Fire does enough damage to consistently heal, and it ticks off at a decent rate and for a good amount of times during the spell's duration. Not to mention the fact that duplicating the beams with this Soul Mirror trick is a literal damage multiplier for no additional cost other than having to move your monks around and taking some initial damage for them to create more beams. The more monks with Soul Mirror you have in your party, the more beams you can create for additional amounts of healing and damage. If you also cast Combusting Wounds on your enemies, you can deal even more damage from the rapid-fire procs coming from multiple beams.
Another part of this interaction you can exploit is the fact that the reflected beams are making attack rolls against the wizard but not harming them due to their fire immunity. So you can use it to trigger items that have on-hit effects while safely not taking damage. For example, wielding the Lover's Embrace dagger, upgraded with the Lover's Quarrel enchantment to match your wizard's gender, can give you Frenzy when you crit yourself with Ray of Fire, and if you also have Slayer's Claw in your offhand, you can get +2 penetration from the upgraded might inspiration. Acolyte's Frostbite has an enchantment called Vengeful Cold which creates a frost blast upon getting crit, however it is tricky to utilize here because it requires you to be a minimum distance away from your allies before it will trigger and its attack will be directed towards the monk whose beam triggered it. The Effort with Hemorrhaging trick to turn your beam attacks into melee attacks also works here, but I haven't found a good way of leveraging this yet. I tried using it with Minoletta's Piercing Sigil and Okura's Kettle, thinking I could trigger their AoE effects, but they unfortunately didn't work. I think it's because they need the attacks to come from an enemy and not yourself.
If anyone is able to come up with ways of benefitting from nondamaging self attacks (which can be melee coded if you also use Effort), please share!