Many spells allow a saving throw to avoid or mitigate spell effects. This saving throw is a property of the spell target that represents how good they are at throwing off the effect of magic. Thus, it is in effect a magic defense stat (much like the “will” defense in 4E).
Now, I already have magic-users roll a saving throw when they cast a spell to see if the spell is retained. Why not roll the retention save and the monster save together? One minor problem is that the player wants to roll high on the retention save, but wants the monster to roll low on the defense save. However, this is easy to address; just subtract the monster save from 20 to get a new target number.
For example, say the target of a spell has a saving throw versus magic of 15. That means they have a 30% defense against magic. Also assume that the caster has a save versus magic of 15 (and thus a 30% chance to retain the spell). The player rolls 1d20. Above 5 and the monster is affected (30% chance preserved). 15 or higher and the spell is also retained. This is a quick and easy single roll spectrum system that uses all the default game numbers.
The one minor hack that I would add is to have the magic-user apply spell competency* as a bonus to the roll and spell level as a penalty to represent spell difficulty (at first and second class level these modifiers balance out, so no math is required until a magic-user reaches third level). This makes the all-in-one saving throw more like a direct “spell roll.”
* Spell competency = the highest level of spell that the magic-user can prepare. This is usually equivalent to experience level divided by 2 (round up). For example, a fifth level magic-user has a spell competency of 3.
This also has the side effect of preventing the M-U from retaining the spell if it does not affect the target.
Also, how would one adjucate the same spell affecting multiple targets? Either they all are subject to the spell or none of them are?
Not necessarily; depending on the target save and caster level, the two thresholds might be inverted (for example, it might be easier to retain than it is to affect the target). However, you are probably right in most cases. I think it fits the idea of a casting skill check though: if you cast it well, it affects the target, and if you cast it really well, you can use it again. I could see how someone might want to separate the two chances though, and that would be totally reasonable (of course, that does require two separate die rolls).
For multiple targets, my inclination would probably be all or nothing (again following the logic of the casting skill check) though different targets might have different save numbers. Thus, a fireball might toast all the goblins but be partly resisted by the ogre.
I suppose one could also do as 4E suggests and roll per target, if desired (using the first roll for retention, or rolling that separately).
You also run into issues at higher level when the threshold do inverse however. Say for example that the monster saves versus spell on a 8 or higher, and the the M-U retain the spells on a 9 or higher. So from 1 to 8, the monster is unaffected and the spell lost. From 9 to 13, the monster is unaffected, but the spell is retained, and from 14 to 20, the monster is affected and the spell retained. there is no way to affect the monster and lose the spell.
Now that isn’t so much an issue if the skill of the spellcaster is what determine if a spell succeed or not but it doesn’t make much sense if the monster has a seperate resistance. I guess this is just a case of different paradigm for magic.
Your one roll spectrum is quite elegant when you picture magic the right way.
I can see the elegance of what you’re aiming for, here. (And can’t help but wonder if my consistent failure to remember the spell retention save has prompted it!) But to my mathematically challenged mind, this seems more complicated, rather than less.
Well, the extra math is mostly on the referee side. So, for example, you roll your save (say you get 14). You know your save target number for retention, then you just ask the referee whether 14 is good enough to affect the target.
The spell competency modifier might be too fiddly for playing over hangouts, but I think the system would work fine without that too (as the saving throw naturally gets better with level anyways). I just like the idea of modelling spell complexity and difficulty so that lower level spells are easier to retain than higher level spells.
As I just realized, armor worn should of course penalize this magic roll.
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