How common are creatures capable of doing magical damage in 5e?

Question

I've been thinking about homebrewing some things. One of the benefits that I've been considering is a resistance to non-magical damage (for example, for a sort of Rage-lite feat), or perhaps immunity (for a sort of door-barring spell). I thought this was a neat idea, because it gives the DM the ability to override it if they need (give important baddies magic weapons), but also gives it a high cost (the party gets to loot a magic weapon if they have to contend with someone who cancels this advantage.)

But, thinking about it, there might be a lot of monsters with magical attacks available. Or maybe attacks from non-humanoid, non-beasts are magical by default or something. If either of those were the case, that benefit doesn't do anything in most cases. So, How common are magical attacks (including ranged/melee/spell, in terms of quantity of monsters in released sources (Monster manual, Volo's guide, Mordenkainen's Tome, etc.)?

A good answer would give a general idea of whether this benefit would apply often enough to be useful in the way I hoped (enemies that are unaffected are generally special) (e.g. "As a DM, I find that nearly all/very few of the monsters I use have a way of doing magical damage"), but the best answer would have actual numbers.

I'd love to have a nice table that just lists out these things (like something that says by CR ranges, here's how many have no magical attacks, here's how many whose attacks count as magical, and here's how many have spells, or whatever categories make the most sense), but I don't know if there are databases that tabulate information like that (Could a search for "counts as magical for the purposes of overcoming resistance", or the like, work?). Perhaps a random sampling of a more manageable number of monsters could be considered representative?

Answer 1

A few major points:

As for the commonality of magical opponents, it really depends on what level and size your party is. As a DM of (now) level 5 characters, we are barely fighting any 'magical' opponents. but at higher levels, I believe that this would change.

Resistance is HUGE. Especially if your character is a high AC character, disadvantage is a massive benefit. And if you have resistance to all non-magical weapon damage (Which is most of the damage you will be taking), you will be basically indestructible.

As for your DM finding magical opponents so as to balance this, this would just put unnecessary stress on your DM, and greatly limit the variety of opponents that he could use. As well as this, wouldn't it just be better to not have the resistance and be faced against a greater variety of opponents?

TL;DR - Magical mobs and enemies are in general, very rare, and difficult for the DM to use. Having resistance to non-magical damage would be supremely overpowered and in general just a bad idea.

Answer 2

As a starting point for homebrewing effects, you may want to consider some of the existing effects that reduce damage:

• Heavy Armor Master, feat, grants +1 to Strength and reduces bludgeoning, slashing, and piercing damage from nonmagical weapons by 3.
• Protection from Energy, 3rd level Concentration spell, grants resistance to one of acid, cold, fire, lightning, or thunder.
• Stoneskin, 4th level Concentration spell, consuming 100 gp worth of diamond dust, grants resistance to nonmagical bludgeoning, piercing, and slashing damage.
• Flame Shield, 4th level Concentration spell, grants resistance to fire or cold in addition to other benefits.

From these examples, we can gather that the designers of 5e consider damage reduction to be a fairly strong effect, costing an ability score improvement, a 3rd or 4th level spell, and in the case of Stoneskin a significant amount of treasure. Also, these effects only apply to some damage types, and Stoneskin does nothing against damage from any magical source, including spells.

Damage reduction against all non-magical damage would be more powerful than Stoneskin, but not as much more powerful than you might think: by the time PCs can cast Stoneskin, a significant chunk of enemy damage output could easily be from opposing spellcasters, with melee combatants mainly acting as a screen to keep the casters out of melee long enough to get their spells off. On the other hand, damage reduction against everything but magical weapons would be very powerful, better than Stoneskin plus every variation of Protection from Energy; I would be extremely cautious about handing out such an effect.