Magic Resistance

Magic Resistance? Isn't that like, a save versus spells?

Not exactly.

One of the huge improvements AD&D brought over D&D was the concept of magic resistance. Before a monster made a saving throw, you check the monster's magic resistance score. If it was standard, fine, nothing happens, and the monster makes a save versus spells as normal.

The Succubus, as seen in Swords & Wizardry

But if a creature had magic resistance, you rolled that first before the save, such as a succubus having a 70% magic resistance (don't believe the 1e Monster Manual PDF listing of 10%, lots of 1's and 7's got swapped, check the DMG's monster summary lists and you will see it as 70%). You roll 01-70, and the spell bounces off - no matter the level or if it is a wish, or what - the spell instantly fails, and no save is needed.

Eliminating magic resistance is a huge gift to spellcasters and a huge nerf to martial DPS.

And I can't stress this enough, there may be times when all your cleric, druid, illusionist, bard, and wizard magic just doesn't do a thing to the monster you are facing, and you are out of luck unless your fighters and rogues can smack the critter down. This was a nerf to the overpowered spellcasters of D&D when AD&D came around, and it helped even the playing field and make strong martial DPS classes useful in a party.

Labyrinth Lord, B/X, and Old School Essentials? No magic resistance.

Swords & Wizardry, AD&D, and OSRIC? Magic resistance. Adventures Dark & Deep has magic resistance based on a level-of-caster-based chart.

D&D 3/3.5/Pathfinder 1e call it Spell Resistance (d20+level to overcome). Castles & Crusades has spell resistance, but it is a flat d20 to overcome, with no level modifier (closer to AD&D).

D&D 5? No magic or spell resistance, and some monsters have an advantage on magic saving throws, which is clearly not the same thing (and ironically closer to B/X). Our succubus does not even get an advantage to magic saves, and they demoted her out of the demons category, which kind of sucks.

Of course, B/X does not have the magic resistance concept, but AD&D 1e does. You can see the clear lineage of the AD&D-based games and this is one of those key improvements they made to reduce the incredible power of spellcasters and make martial DPS valuable again.

I love the simplicity of B/X and the ease of use, but some decisions Gygax made when creating AD&D were improvements and addressed real balance issues. You just can't get 20 low-level magic users and kill Orcus with volleys of magic missiles.

We tend to elevate games and designs on pedestals with our nostalgia, but if you go back (and you were there at the time and remember) you will see some of the same issues we had back then happening today. And in our rush to embrace simplicity, we tend to brush aside the real improvements Gygax made to his game as an evolution of the rules.

And I feel a good one that addresses the chronic issue of unchecked caster power.