Is a monster having immunity to non-magical weapons negating the concept of not strictly needing magic items? Or is the intent to force players to get creative when facing such monsters?We believe that not all monsters need to be able to be defeated in a straight, head-on fight. Some monsters should require players characters to either have the right tools for the job (in this case, magic items), or be creative in how they deal with them. We don’t want the answer to every monster problem to automatically be “stab it until it dies,” and that goes for spellcasting, too; there may well be monsters that end up in the game that cannot be harmed by spells. We think this is good for two reasons: one, it makes having the right tool for the job (and the tool itself) much more special and valuable, and makes the player feel good for having it; and two, it adds texture and problem-solving to adventures rather than encouraging players to simply barrel through every adventure using violence as the only solution.All that having been said, any time we deal with something as impactful as immunity to spells or immunity to nonmagical weapons, we have to be sparing with how often we sprinkle it throughout the game. We don’t want to end up in a situation where those monsters that provide that texture become the standard, creating a reversal where players are frequently frustrated by having to constantly deal with monsters that cannot be harmed by traditional means. Like all things that present a non-traditional challenge to the players, we need to be judicious in their use so that they retain their value as an exceptional thing that provides texture, not a constant source of frustration.
All good points on the part of Rodney Thompson (thanks to Haven for the edit. :P), but what doesn't get addressed here is how to use them as a GM. "We" here doesn't seem to imply the GM, after all, but the overall design. And that's fair; they're clearly aware that giving even a significant portion of the monsters huge, sweeping immunities could easily lead to a lot of player frustration. What they don't get into, and what I'd really like to see in a DMG 1, are some basic uses for monsters with weapon/magic immunities, and how to make them fun.
For starters, don't have that be the only kind of monster in a given battle.
This could actually be a very general statement: where possible, include monsters of different types, with different strengths and weaknesses. The NPC roles of 4e were very useful for this, as having a controller, a couple soldiers, and a bunch of minions with complementary abilities often made for complex and engaging fights with a lot of layers. The magic or weapon immune monster could be likewise interesting, but the trick is providing enough stuff for the characters whose go-tos are obsolete something to do.
Thus, my next suggestion is, where possible, include manipulable elements in the combat location, and make sure that the excludes players can use them. In a battle where the big bad is immune to spells, don't have a lot of strength-check challenges, but structural items that spells can weaken and knock into the monster? Pretty awesome. In one where the monster is immune to non-magical weaponry, having a lot of mundane adds, or say... having something like a giant fire-pit where the fighters can temporarily gain a swing or two of enchant, but have to do something to keep it going? Very cool, and gives everyone something to think about while the combat is going on. If you can't tell, I was madly in love with terrain powers, zones, swarms, and special terrain in 4e, and I very much hope to see those come back in D&D next.
I think in general, a basic ideas guide in the main released DMG on how to stat fights for this edition, and how to set up the fights to be entertaining would go a long way in helping everyone at the table have fun with it. At this point, most people playing/testing Next have a lot of expectations and habits built up from previous editions. A little bit of guidance in how to make the most of this one, and how to realise what the designers had in mind for combat structure, would be a very good jumping off place for making it their own.