Trials of Chult
Yeah I use 2nd edition rules which are quite powerful. The way it works is
Spell Turning 7th Caster 1rnd/level
This spell creates a invisible magic reflective barrier. Any spell that is instantaneous will be partially reflected back at caster. The reflection is 1d10 x 10%. This will effect area effect spells like Fireball and Lightning Bolt, but not stationary spells like Cloudkill. Only spells subject to SR are affected. Finally it must be a spell that was cast at you by a caster, not a ward or magical trap.
Some examples are things like Ray of Enfeeblement: lets say someone cast this at you and makes a touch attack roll. There is no save vs this spell but SR applies. you roll a 5 on the d10 so 50% of it goes back towards the caster. So you take half the strength damage. The caster automatically hits himself if he hit his target, regardless of the difference in Touch AC.
If it is a spell that does damage like Magic Missile or Vitriolic Sphere. Roll the d10 first. Even if you are immune to the spell coming at you (lets say its MM and you have a shield spell up) you can still use Spell Turning to hurt the other guy. If it is a continuous damage effect like Acid Arrow, Burning Blood, or Vitriolic Sphere then the duel damage remains until duration is complete or each victim gets rid of it individually. So if you get hit with a acid arrow and roll a 3 for 30%, and the other mage also gets hit with the spell. Then every round roll the acid damage, with you taking 70% and the other mage taking 30%.
If its an area effect spell like Fireball, it depends if it was targeted directly at you. If it was then the entire Fireball spell is subject to the d10. If it was just targeted near you then only the damage that effects you. Lets say you are in the middle of your companions and the enemy mage (in the middle of HIS companions) throws the spell directly at you. roll a d10. lets say you roll a 2. I roll damage and come out with 33. 20% of 33 is about 6.5 so we round up and say 7. So a 25 (33-7) fireball washes over you and the Four Winds. At the same time a 7 fireball washes over the enemy and his group. So one fireball spell effectively hit every combatant. Now lets say the same fireball was targeted at Zena who is standing next to you. And again you roll a 2. Zena and anyone else nearby have to save vs a 33 point fireball, you have to save vs a 25 point fireball, and the mage who cast it (and only him, not anyone nearby him) has to save vs a 7 point fireball.
Finally, what if they cast a nondamaging spell like charm person? Roll the d10 and that is the percentage that bounces back. Lets say you roll a 9. So close to all of it! You have to save vs the charm person but you have a +9 to do so. The mage has to save against his own spell with a +1. It is theoretically possible that both of you fail and you are both charmed at the same time.
Updated 9 – 25 – 11
Spell Turning mainly targets spells. So lets start with that.
1 Spells targeting you specifically: Example – Magic Missile, Harm, Finger of Death, Bestow Curse, etc.
They only target one individual. The hard and fast rule is “If Spell Resistance affects it, then so does Spell Turning.” There may be exceptions to this rule but I cant think of any off the top of my head. If the spell requires a touch attack (Ray spells, Acid Arrow, Essodrens Lightning, etc) then the hard and fast rule is if the caster manages to hit YOU, then they also hit themself, regardless of the difference in your touch ACs.
So if your tied up and helpless but still wearing your ring. Touch AC 5. And a mage cast a Ray of Enfeeblement at you or something, then when I hit you, I hit myself even if I have a Touch AC of 50!
2 Spell targeting an area that you occupy: Example – Fireball, Light Bolt, Cloudkill, Acid Rain, Meteor Storm
Ok this is where it starts getting a little tricky. First, “Does SR affect the spell?” If the answer is NO, then Spell Turning wont do anything. If the answer is yes then the next question is “Is the spell instantaneous?” Spell Turning only works on instantaneous spells. Lets look at the above examples.
Fireball, Cone of Cold, Air Lance, Lightning Bolt, Burning Hands – all these work the same way. They are all affect by Spell Resistance and area effect damage instaneous. If I target you specifically then your Turning roll affects the entire thing. Lets use Fireball for this example. I throw a fireball at you for 50 points of damage. You roll a 1 on d 12. That means 10% (5 points) is thrown back at me. Two 30’ diameter fireballs explode around both you and my guy. Each of you makes a Reflex save as well as everyone around either you or me. Around you for a 45point Fireball, around me for a 5 point Fireball. If you had rolled a 9 then it would be the same thing only you would have the 5 pointer and I would have the 45er. If you roll a 10 or higher then there is only one 50point fireball centered on my guy.
Cloudkill, Stinking Cloud, Acid Rain, Fog Cloud, Incendiary Cloud, Prismatic Wall, etc – all work the same way. They are area effect continuous spells. Some allow SR, some dont. But Spell Turning wont work on any of them.
3 Spell like effects: Example: Drows globe of darkness, Dragons breath, Hexblades Curse, etc
This is the toughest one because its almost on a case by case basis. The way I do it is, if the effect is basically a spell (like a drows globe of darkness) then we treat it like a spell. Darkness isnt affected by SR so no Spell Turning either. Now some abilities match spell descriptions pretty close. There have been so many 3.5 spellbooks written from so many sources that pretty much anything imaginable is in print somewhere. Look at Marks spells for Essodren, most dont come from the PHB. Im actually ok with this unlike alot of other DMs because I have made up quite a few unique spells for my campaign myself.
You could easily argue that a White Dragon breathing is like a Cone of Cold, or a Blue Dragon would breath Lightning Bolt. Both of those are vulnerable to Spell Turning. But they are breath weapons the creatures can do naturally as bodily functions and are not vulnerable. When dragons get older they can cast certain spells so many times per day depending on their subspecies. These spells act like magic cast from mages. I guess the easiest way to describe it is if it would still work in a magic-dead zone then it goes through Spell Turning.