Magic Resistance

Speaking of Swords & Wizardry, I noticed something peculiar about the rules: it borrows the magic resistance mechanic from the second edition of Advanced Dungeons & Dragons, rather than the rule stated in OD&D or the first edition.

On page 92 of Swords & Wizardry Complete, magic resistance is described as follows:

One of the "Special" items that may be listed for some creatures is "Magic Resistance," followed by a percentage. The given percentage chance is the likelihood that any magic used against the creature (other than bonuses from weapons) will fail to take effect. Roll d100 and if the result is less than the given percentage, the magic will fail.

In contrast, the rule in OD&D and the first edition of Advanced Dungeons & Dragons differed. As stated on pages 5-6 of the first edition Monster Manual:

"Magic Resistance indicates the percentage chance of any spell absolutely failing in the monster's presence. It is based on the spell being cast by a magic user of 11th level and it must be adjusted upwards by 5% for each level below 11th or downwards for each level above 11th of the magic-user casting the spell. Thus a magic resistance of 95% means that a 10th level magic-user has no possibility of affecting he monster with a spell, while a 12th level magic-user has a 10% chance.

The same rule was stated in the entry for the Balrog in OD&D's Monsters & Treasure, but was omitted when the Balrog was deleted from subsequent printings.

The mechanic used in Swords & Wizardry did not appear until the second edition of Advanced Dungeons & Dragons. For example, page 66 of the second edition Dungeon Master Guide states:

Magic resistance is given as a percentile number. For a magical effect to have any chance of success, the magic resistance must be overcome. The target (the one with the magic resistance) rolls percentile dice. If the roll is higher than the creature's magic resistance, the spell has a normal effect.

It is interesting that Swords & Wizardry adopts a second edition mechanic in no small part because second edition's reputation in the OSR is mixed at best. It also is notable because Swords & Wizardry Complete often makes clear when it is departing from the original rules or offering an interpretive gloss on those rules; for example, see pages 27, 34, 36, 40, 41. It does not do so with respect to magic resistance though. So no explanation is offered for why this choice was made.

On the one hand, the second edition mechanic has the virtue of simplicity: no math necessary, just a straightforward percentile roll. But it also plays differently, particularly if one retains the same basic magic resistance statistics for monsters. (With one exception, Swords & Wizardry states the same magic resistance statistics for demons that were stated in the first edition Monster Manual.) Under the OD&D and first edition rule, magic resistance scales; as magic-users increase in level, they become more likely to overcome a creature's magic resistance; under the second edition rule, the magic-user's level makes no difference. This has the effect of making creatures with high magic resistance more powerful on balance.

I don't prefer one mechanic over the other. But if I used the second edition one, I'd also modify the magic resistance of creatures from those stated in OD&D and the first edition. I'm surprised that Swords & Wizardry did not.