What would happen if characters gained resistance to spells proportional to their martial level?

Question

I am considering the potential implications of a house rule for Dungeons and Dragons 5th Edition. The rule is as follows:

1. A character's "Martial Level" is calculated by determining their spellcasting level using the multiclass spell slot rules (Player's Handbook pg. 164), then subtracting that level from their character level. For example, a 10th level Barbarian would have a Martial Level of 10, a 10th level Paladin would have a Martial Level of 5, a 10th level Eldritch Knight Fighter would have a Martial Level of 7, and a 10th level Wizard would have a Martial Level of 0.
2. When a character would be affected by a magical effect, they can choose to attempt to resist the effect. Their player rolls a d20. If the result is equal to or less than their Martial Level, the character is not affected.

How does this proposed house rule interact with the existing rules as written? Does it produce any ambiguities or counterintuitive results?

Answer 1

You ask, "Would this change have the intended effect of improving the capabilities of non-spellcasting characters?"

By definition, yes. Giving characters resistance to magic in proportion to their martial level will clearly improve the capabilities of non-spellcasting characters.

You ask, "What other side-effects would it have?"

Here are a few:

High level martial characters will easily dominate fights that are otherwise balanced.

For instance, a 20th level barbarian or fighter faces 20 beholders. There is literally nothing the beholders could do, other than bite. Mind flayers, dragons, liches, every challenging monster depends to some extent on magic. All of them will fall before the might of the barbarians and other full martials.

Furthermore, there's nothing in your system that keeps the full martial from using magic. A 20th level barbarian or fighter with a few tasty magic items would be terrifying indeed.

PCs are likely to eschew non-martials.

Certainly full and even partial casters become a lot less interesting, because with high resistance to magic, the martials rule the battlefield. A non-full martial will be subject to magic that their martial peers are less subject to.

Another solution

If you think the martial characters are unbalanced compared to casters, look at the magic items the characters have access to. To me, playing a high-level fighter with cool weapons and armor, and some high-level caster companions, sounds a lot more fun than just being able to beat the monsters because their magic just doesn't work on me.

Answer 2

This will not fix your issue and will make PCs too strong

I think you basic assumption is flawed. You state in the comments

I consider characters who don't have the ability to use magic to be at an inherent disadvantage in a world where people have the ability to use magic.

And I can see an argument for this when it comes to out-of-combat capacities, where spellcasters can do lots of insane things at higher levels that mundane characters cannot, like raising the dead, teleporting, scrying from afar, summoning powerful minions to do their bidding, traveling to other planes, and so on.

But from a power level, especially for combat the classes are pretty balanced, maybe with a tendency of later published classes being more powerful.

Sub-Classes commonly seen as the most powerful are always going to be somewhat subjective based on play experiences, but include Twilight or Peace Cleric, Chronurgy or Divination Wizard, Eloquence Bard, Moon Druid, Gloomstalker Ranger, and Echo Knight. Vengeance Paladins also get some mention as do Hexblade Warlocks, but mostly to enhance multiclass builds. (Here‘s another survey, and here my own experiences with a Gloomstalker Ranger outclassing a Divination Wizard).

While there is an overweighting of full casters (bard, cleric, druid, wizard) there are also partial casters (ranger, paladin) or martials (fighter), so it is not that casters are always better. The maybe more important point is that it is overpowered subclass features, not just the ability to cast spells, that push you up there. And these often are pro-active abilities that make these subclasses strong: the ability to double up on healing, to just beat charisma-based tests, to force through save-or-suck spells, to dish out massive amounts of damage.

Blanket enhancing spell immunity to non-casters is not going to fix these imbalances. It will make martial characters a lot more attractive, but guess what: now you get Echo Knights and Gloomstalker Rangers that already were overpowered, even more overpowered.

This is a strong incentive for players to pick martials, but the game is not necessarily designed to be played without casters. Not that it cannot be done, but it is going to be cumbersome. So someone likely still has to play a druid, bard or cleric so the party has a healer, and that player will feel as if they got shafted.

Also, this is different from normal Magic Resistance, that just gives you flat Advantage on the saving throw against magical effects. Here, you gain an increasing degree of immunity as the PC level progresses, up to total immunity for non-casters at 20th level.

And immunity to all magical effects, which this amounts to at higher levels, is pretty unbalancing. The game assumes this does not exist. What happens if a no-save teleport effect hits the party to get them somewhere and now you have a split party, because some made that roll, and others did not?

Lastly, and most importantly, this just makes the PCs stronger, with nothing to offset it. The game is already too easy on higher levels. Our DM routinely confronts our group of 3 high level characters with challenges that have multiples of the XP budget that is "deadly" according to the DMG for a group of 4. This would just make matters worse. The monsters gain no such immunity to the PCs spells.

D&D is designed to have a high level of powerful spellcasting. It is going to be very difficult to change this with a simple mechanical hack and not disbalance the game in some way. This is no fault of yours, it is just hard without a major reworking of the game. Maybe a better solution for you, if you look for something where martials play a leading role, or magic is lower-key would be to pick a different game system, such as Pendragon or Runequest.

Answer 3

What would happen is that the game balance would go completely out of the window.

5e is in my experience better balanced than any other DnD (or pathfinder for that matter) edition. In fact it doesn't have any serious issues in any level range (I have played and run adventures in low, mid and high levels). Casters may have more out of combat utility than martial classes, but then again, the focus of martial classes is in martial things - combat. As it is a good fighter will outdamage a caster through the adventuring day. A bad fighter will outdamage a caster in a combat against a single strong foe - because that is the strength of the class. So it is really debatable which class is stronger: wizard or fighter.

But if you introduce your rule, then classes like fighter, barbarian, rogue (an interesting case when the class HAS a good out of combat utility) will completely overshadow other classes. A good deal of high level threats would be powerless against them, so preparing a challenging encounter will become a chore. It should be a hint that the change is a bad idea when encounter become easier if a fighter attempts it solo over if a whole party engage the enemy.

Also, a bit of a frame challenge:

I think your reasons for the change are deeply flawed.

1. The General Problem of Mundane and Magic Rules

I completely disagree with this part. DnD is not a simulation. Gravity doesn't work by using physics rules, but by the rules as written in a PHB/DMG. So premise that "Rules that model mundane things are subject to "reality checks" on the part of the referee and are therefore generally an upper bound to what a character can do without magic." is flawed and not at all RAW/RAI. Nor is access to magic "inherently advantageous", as characters with access to magic are balanced by the lack of other things. Sure, wizard has spells, but doesn't have 4 attacks/round like a lvl 20 figher has. Nor does it have whole bunch of skills like a rogue possess.

2. Magic Does Everything

Jack of all trades, but master of none. Sure, magic can unlock a chest... but a skill does the same for free. And better at that, because it doesn't produce a loud knocking sound. In fact, magic can never compete with skills in things a skill can do: tracking, stealth, thievery, social stuff,... Those things can be done with magic, but not nearly as good as with skills. Also, you don't need magic to remove a magic! In my experience the best way to beat a wizard is to beat him to death in his sleep... Something a rogue is way more suited for than casters.

3. Resources Are Not a Balancing Factor

Well, they should be. If you have only one combat encounter/adventuring day then it is no surprise that you have an issue with game balance. Game is balanced on having 5ish encounters in a day. If a party attempt to rest in the middle of a dungeon... well, that is a perfect opportunity for random (or not so random) encounters. If they leave the dungeon to rest, well, such actions should have consequences as well.

4. Magic Items Are Not a Balancing Factor

5e is balanced around having little to no magic items. So no, it is not necessary to have them in high level play. You are not suppose to buy them after all. Also, martials are way better at using them in combat than casters. You CAN increase the damage output and survivability of martials with items quite easily. But there are not all that many items that can increase the damage output of casters.

5. The Problem, In Summary

Having magic shouldn't have a penalty. Just like having multiple attacks doesn't have penalty. Or having 8 skills. The balance is that every class gets a limited number of those benefits.

Suggested solution: In my experiences people who are under impression that casters are superior in every way over martials (in 5e that is) run too few encounters between rests. So casters can go nova in literally every encounter. No wonder that casters outshine everything in that case.

So the obvious solution is to run more encounters. If a party attempt to rest after every encounter it is on you as a DM to fix that. You kinda doesn't make a lot of sense to take a long rest at 10 am. Or in the middle of a dungeon.

The other issue is the out-of-combat utility. Make more skill challenges. Magic can solve some issues, but just barely. Skills are in the game for a reason, and some martials are great at using skills.