I know it has been awhile since I mentioned it, but here is the first installment of my thoughts on magic systems for use with USR (Unbelievably Simple Roleplaying), with an eye towards emulating D&D b/x.
Specific Spells as a Specialism
This is actually fairly simple to explain. If you want to cast a spell, take that spell as a specialism.
Example: Magic Missile (Wits +2), Read Magic (Wits + 2),etc.
When I first read USR, this is the method that jumped out at me as a logical application of the system of specialisms. In the case of Magic Missile , it's a ranged weapon attack and treated as such, whereas Read Magic would be an uncontested challenge roll with a difficulty established by the GM.
And it plays well this way. I have had a lot of fun with this method.
Unfortunately, this means a Magic Missile and a Fireball have the same possibility of hitting and cause the same amount of damage.
Rather than call this next section another method, I'll call it a slight improvement:
Specific Spells as Specialisms: Specialism Independent Effects
Let's start with an easier example, Cure Light Wounds (Ego +2).
In this case, the first few times I did this, I went a little farther and decided this should be an opposed roll - Ego of caster vs. Ego of recipient. If the recipient loses, they gain 1d6+1 hp, regardless of the difference between the two results.
So, back to Magic Missile.
The attack is made against the target as (Wits+2). The target, incidentally, rolls defense as normal in my games, but I do know that in b/x at least, the hits are automatic (and ultimately, for a conversion, they would be. As these were for one off games, I wasn't yet considering correspondence to b/x). If the target loses, then damage of 1d6 per missile (1 missile per +2 of specialism), regardless of the difference in the two results.
Fireball could then do 1d6 points of damage for the first +2 and an additional 1d6 for each bonus thereafter. I.e. if the magic-user had Fireball(Wits+3), they'd roll 2d6 for damage. Now the fireball and the magic missile distinction isn't meaningless.
We're getting closer. But it still feels not quite right.
Why This is not a Good Approach for B/X Emulation
While the benefit here is that the caster's level doesn't in any way impact which spells they can choose as specialisms, it's not an emulation. For one, spells have levels in b/x and only one of those levels are accessible to a magic user just starting out.
And, in all cases, the approach I've presented here turns magic into a skill check, but a magic-user in b/x is always assumed to cast their spell successfully (unless interrupted). So, although this mechanism works fine generally, in an attempt to emulate b/x (warts and all), it's the wrong choice.
A Better Emulation:
Use the specialism Magic(Wits +2) for magic-users/elves and Magic(Ego +2) for Clerics.
Use the spell lists available for b/x (Labyrinth Lord is free and has the same spells more or less). Yes, spell lists can be annoying in a rules-lite game - especially when you can't remember the effect and have to look it up mid-game. But, they are, I think, an expected feature of any game trying to be d&d-like (which is my goal here after all). The work is in converting the spells you want to use to USR (and it's not hard work at all).
Casting is automatic unless interrupted.
Given how advancement in USR is handled, I would lean towards something like one spell per Magic bonus (+2,+3, +4, etc.) of a level equal to or less than the caster's level. This doesn't exactly correspond to b/x but it's a decent enough compromise that doesn't make for overly powerful magic-users.
So, a 1st level magic-user with Magic(Wits+2) can cast 2 spells of first level. At 2nd level, assuming they bump Magic up 1 to Magic(Wits +3), they can cast 3 spells of first or second level. At 3rd level and Magic (Wits +4) they can cast four spells of third level or lower, etc.
Clerics have no spells until second level, per b/x and then they get 1 per point. So they go from none to 3. It's a big jump but prevents adding yet another system to the pile and preserves the idea that they are capable fighters and not armored nurses.
Spells that hit automatically, do so.
Spells where the victim gets a saving throw, the GM decides the difficulty involved (corresponding to the presumed resistance of the recipient to magic) and roll an uncontested check using the appropriate attribute (Wits for magic-user spells, Ego for cleric).
Utility spells, in my opinion, should have a chance of failure - but that's not a b/x feature. So I'll just leave you with that thought.
The above has not been play-tested as well as I would like(few of my magic-users ever live through their first outing to test the advancement rules) and I make no guarantee it will give the desired effect, but it's a start in any case.
You might suggest that the rogue has to choose which particular abilities they want but the magic-users / clerics are getting a boon by being able to choose a very general skill. I think this is true to a point, but I think in terms of capturing the feel of b/x, the rogue method I proposed in my last post on the subject works well for that class.
The thief's (er uh rogue's) abilities improve with level in b/x (as they do in USR), but a spell caster ability to successfully cast a spell does not increase with level (with maybe a handful of exceptions), although the choice of spells does. Thus, while specific rogue skills map quite well to specialisms and how they function and how at least I think of them, specific spells do not.
*For Giggles, Here's a System Not to Try
Specialisms of Attack Spells(Wits+2), Defense Spells(Wits +2), Utility Spells(Wits +2). I only allowed my character to choose Utility and then either Defense or Attack.
I chose Defense. Total Disaster.
It's certainly not a good approach for an emulation but it can be fun when that's not your intent.