100g worth to cast a concentration spell that only lasts an hour, and doesn't protect against magical attacks... seems under-powered to have such a high cost to me?

I'm playing a Goliath Way of the Four Elements monk and this spell seems to be a really themed choice to take as "Eternal Mountain Defense", however I could take this only one level before I can use KI to turn invisible and since resistances doesn't seem to stack...Aside from the duration, what's the point of  this spell for a monk anyway? I guess I coudl take it, shirt rest and still have 30 mins or so before it runs out...

For some clarification, an ancient dragon's melee attacks are not magical, though the elemental damage would be unaffected. Giant's do not have magical attacks. A Death Knight's melee attacks are not magical. The Tarrasques' melee attacks are not magical.  I mean, 100gp to have resistance to all the Tarrasques melee attacks? The spell is perfectly fine, and like many spells, very situational. Against some creatures, it won't do much to help. Against others, it can turn the tide of battle. 

No this spell is pretty bad. Resistance to non-magical  bludgeoning, piercing, and slashing damage at the cost of concentration are the main issues. For this spell to be of any use at all it necessitates that you get hit a few times and mitigating those attacks to minor damage, or mitigating the effect of a heavy damage attack.

As a caster being hit is sort of opposite to your game plan. There are exceedingly few circumstances where it would behoove you to sit there and tank damage you think is incoming as opposed to getting out of dodge with a misty step/ thunder step/dimension door, or even a simple Dodge or Disengage action. You are simply too squishy, and likely have a poor to average Con Save making a single concentration check very risky. Multiple checks or high damage and thus high DC concentration checks are next to impossible. Not to mention this is what you are concentrating on limiting a lot of your most powerful spell selection options. All around bad. 

As a martial that is high enough level to cast this, you likely have a great Con save and tanking is probably your intention. It's just disingenuous to assume that in any given encounter an enemy is going to rely on non-magical  bludgeoning, piercing, and slashing damage. i.e the dragon with the breath weapon. Anything else that isn't as dumb as a rock is going to just target your squishes seeing you tank their damage like that. For the few things that are dumb as a rock and are strong enough to tangle with a high level martial and have no damage alternatives. They are going to have a seriously gnarly amount of attacks and/or damage to compensate. i.e the Tarrasque.

The most effective use of this spell I see is as a buff to an ally. Casting this on a party member around the levels when you can first cast it are somewhat helpful. The caster can preemptively cast this before getting into a fight or on the first turn, and then retreat to a safe spot. However even at 7th level a party will be running into plenty of enemies who do not rely on non-magical bludgeoning, piercing, and slashing damage. And there are better buffs. As far as defensive options go Mirror Image can turn 3 hits into a miss. Avoiding damage > halving damage all day. 2nd level spell no concentration. Haste is the easy one for your allies. But on oneself I would even say something like a Fire Shield would do more to deter an enemy. Your choice of resistance to one of two pretty common energy damage types and a nasty consequence for hitting you. For the martial that can cast it would really ramp up some damage output if they expect to be hit a lot. Cherry on top it also doesn't require concentration. 

The best case scenario is if for whatever reason one knew in advance they were going to take some non magical bludgeoning, piercing, and slashing damage and they wanted to reduce it. Triggering a trap or jumping off a building are the only things that come to mind. A pretty hefty cost of a 4th level slot but I guess it could save your life.

I don’t think it costs concentration.

This spell seems REALLY strong. There aren’t many ways to get resistance for player characters. Even heavy armor mastery only gets you a 3 damage reduction. This spell has to be expensive otherwise it will seriously infringe on a corner feature of the barbarian.

Honestly coming from an edition (2nd) where the spell negated all damage from 1d4+ a number depending on your level it was a tank of a spell every caster that could took. It was what allowed them to survive against a lot of things.

Now based on the mechanics in this edition work that set up would be very over powered and would not mesh well with the system so I get the change... but is a major downgrade in terms of protection ability made all the worst by concentration being needed. Unless you built your character with concentration saving throws in mind, there is a fair chance of losing it per hit. Additionally there are plenty of creatures in the CR 5 - 10 range that have empowered attacks which negate this fully and those that don’t can hit hard (giants come to mind). To add insult to injury there is a potion you can craft (if your DM allows it) at this level which grants resistance to all damage without requiring concentration and the Barbarian effectively gets a better versions of this spell at 1st level. 

At a minimum for this spell to be usable it would need to last an hour without concentration, then I might actually have to think before choosing something over it.  As it stands now it is a waste of a 4th level slot

What are you talking about, moralhazzard333?
It *does* require concentration. It says up in the spell description very clearly.
And there are several ways to get resistances. Resistance isn't a barbarian unique thing.
Tieflings with the racial feat can get resistance to 3 damage types at base. Rune knights using the hill rune can get resistance to bludgeoning, piercing and slashing.
There are plenty of magic items that give resistances. Kalashtar have psychic resistance. Spells can give resistance and even some subclasses have given it.

Where is this "seriously infringe" on the barbarian?

This is the slightly improved version that we use in our group: as written, except: " Until the spell ends, the target's AC can't be less than 17, regardless of what kind of armor it is wearing, and it has resistance to bludgeoning, piercing, and slashing damage."

Spell sucks for 4th lvl

I don't know why this thing has a 100gp cost. It seems perfectly balanced without it. You can compare it to Investiture of Stone, which is 6th level, has the same effect plus two other great effects, 10 minutes concentration and no gp cost. It's self-only though.

It does cost concentration, it's there in the description.

If I cast this on a Raging barbarian, would that give it half damage, over the half damage it would take from Rage?

No, resistances don’t stack.

I think even blade ward cantrip could resist magical weapon damage.

An ancient red dragon deals 26d6 damage with its breath attack. Lets be cynical and assume the druid fails the dc 24 dex save. On average they then take 91 points of damage, for a concentration save that they cannot make. The spell is lost, for no value. The concentration kills this spell.

Quote from McMuffles >>

An ancient red dragon deals 26d6 damage with its breath attack. Lets be cynical and assume the druid fails the dc 24 dex save. On average they then take 91 points of damage, for a concentration save that they cannot make. The spell is lost, for no value. The concentration kills this spell.

The benefit of this spell is not to give it yourself, but to others; concentration is less of a problem if you pop it on a tank who then takes the hits instead of you, as you only test for concentration if you are hit.

A barbarian party member won't need it, but anyone else will benefit greatly; a Fighter for example can go from a character that can already take a tonne of punishment into one that can take potentially twice as much.

I do agree that the casting cost is a bit of a head-scratcher; similar spells don't have one, I'd drop that but keep concentration and the spell is fine for what it's actually for (allies, not yourself).

Yes, but the dragon's breath attack is an area of effect and an ancient dragon WILL find a way to hit more ppl with it. Also this is a spell avaiable for a ranger and a monk as well, and the monk can only cast it on themselves.
I mean it could be useful, but the fact that it costs money AND concentration makes this a really dubious choice.
And the barbarians damage reduction, especially the bear totem warrior is ridicolously overpowered on later levels.
For reference, the ancient red dragon attacks with a +17. That means that gaining resistance to it's melee attacks is equivalent of having an AC rating of 28.
And this is what could make the spell actually worth it. But what also makes the barbarian really strong, lol.

Please fix this worthless spell.

As a DM, I would eliminate the material cost requirement or make it not consumed by the spell. It is a risky/situational enough spell due to costing concentration and a 4th level slot all the while not doing anything against magical attacks. It doesn't need to drain the caster financially as well. Eliminating concentration on the other hand would probably make it OP. I like that it has to fight against spells like greater invisibility. This way you have to choose between permanent advantage and resistance. Even with the gold cost removed, it is probably a bit weak, but still viable.

"please fix this worthless spell."

"Concentration for resistance is useless because you need to get hit for resistance to be useful and nobody would ever buff their party when they could hog the glory themselves."

"This 6th level spell that lasts has 1/6th the duration, can only target self, and cannot be used by 6 of the classes/subclasses that can use this spell doesn't have a cost associated with it and gets two additional affects and thus eliminates the need for this spell to exist."

You guys should go back to just flinging fireballs at everything.