I'm a new DM. I know that the world is ours to create and we can do what we want within reason, (or not), But i was wondering generally if there was such a thing as items that offer Magic resistance to PCs or NPCs. How much is too much? Thinking of a barbarian NPC that has a hammer that offers 30-50% resistance.
It sounds like what you want, though, is a reduced effect, which usually takes the form of specific elemental resistances (For example Fire Resistance 5). These usually are a flat number subtracted off of the damage dealt, since generally numbers are low enough that percentile effects aren't that variable anyway.
I'm not sure if 4E has Spell Resistance in the same way, but it does have elemental resistances that work pretty much as they do in 3rd. It also adds Vulnerabilities that do the opposite, which can be interesting.
You are certainly able to add an item that gives an across the board resistance, but I would caution you to give the magic users in your party some other way to contribute to the fight, since they could easily get frusatrated by such a formidable defense (this is easily solved by throwing out some minions for them to worry about, or a way to bolster their allies).
Also remember that any item you give to an NPC will probably wind up in the hands of the party after the fight -an item this powerful will shape any encounter you run later, since one member of the party will be able to shrug off an incredible amount of magical damage.
I would recommend that instead you make an item that gives good defenses against the spells your party mages like to use or abuse (Resistance 10 is generally quite good), while leaving holes in the defenses somewhere else. This gives your party an opportunity to explore other tactics or spells they usually skip over, and also leaves you a weakness to exploit when they steal that hammer out of the NPC's cold, dead, fingers.
If you're playing HERO / Champions, this is dead easy - it's Damage Reduction with a few limitations.
If you're playing 4th ed D&D and you just want to reduce damage from spells, then perhaps you could look at a variation of the Brutal weapon. Brutal weapons allow you to re-roll any damage dice that come up equal to or less than the weapon's brutality value - if it's a Brutal 2 greatclub, you re-roll ones or twos on damage dice. The opposite value, IMO, would be something that forced an enemy to re-roll any damage dice that came up equal to or greater than the "anti-brutality" value - an Anti-Brutal 5 would make 'em re-roll fives and sixes. I wouldn't go any further down than Anti-Brutal 5; if they want and deserve anything cooler than that in the same vein, maybe craft one with a daily power to turn a critical hit with a magical attack into a normal attack (but only against you, in the case of magical attacks with several targets).
You could probably adapt that for 3.5 or Pathfinder, but I would have no idea how to calculate what such an item would cost.
Don't laugh, but this is actually for 2nd edition D&D. :)This is practice and prelude for a larger campaign that I want to run with some of my other friends. They will be starting at around 6th level.
The NPC is actually a party member that joined my solo player. I was thinking of having him mimic some of the power of a character from a novel, Karsa Oorlong, from House of Chains who has a natural near immunity to magic. I thought about it and it does make him a bit too powerful and I hadn't thought about the weapon eventually winding up in PC hands. If anything I'll probably pick a school of magic for the weapon to resist, giving a minus to damage rolls or percentile resistance charm spells etc., but I like the idea of a daily power to defend strongly against or nullify an attack.
Its your world. Go for it. I make up my own rules and things all the time. One of my players wanted to be a class that didn't exist so I made it for her for my current campaign. She had a character for Serenity that she loved but only got to play a couple of times beofre the world flopped. When I started my current game I told the players I wanted it to be very open because it would probably be the last campaign I run before I move away for grad school so they could make whatever they wanted for race and class. Anything they wanted to do would be cool. Now the group was rather inexperienced so they weren't people who were getting sick of playing another dwarven fighter, or another elven druid, or whatever.. so they didn't get too outlandish, but she wanted to be an engineer (which unbeknownst to her fit in very well with the theme of the world) in a DnD 3.5 game. it worked out pretty well and she LOVED "the character":http://www.obsidianportal.com/campaigns/the-ilumnos-series/characters/builtlikeatank. Now they are in the last chapter of my game and the setting has changed from a fantasy game with some steampunk elements to a world that is basically Shadowrun. I had to come up with a conversion for much of the Shadowrun elements. One of the players in the group joined up just in this last chapter so he's a cybered up ork. Its really a very interesting mix and they fit well becuase I try not to get bogged down with rules and just know that its my world and whatever I want to exist can and does. (Keeping in mind fairness of course.)
And I bet at least 99% of the people here would agree with that. (I don't think I've met a single "rules stickler" yet on these boards...)
Holy smokes it seems so cool!