I got you, cracks knuckles. (Mind you this will inflict horror, but your barbarian asked for it😉) Environmental damage(traps:  disease, 30 blue slaags (if he is bitten he will slowly turn) lava, bombs, acid rain, collapsing ceilings, poison, home brew CR 30 monsters, separating the party, hurl them into different dimensions, throw 10 beholders at them at once (beholders are my weapon of choice)😈, have them stumble across intellect devourers like 100 of them (statistically characters are gonna get killed and body snatched...if you can body snatch the Barbarian have him slowly butcher his friends.), have an enemy put a bag of holding in a bag of holding (ready made 10 foot dimensional kamikaze trap),use  a False Hydra, and pick off his friends till he is alone and forgets his friends even existed.

Have you looked at encounters that make the party think differently. Target the barbarian with petrify, or have an enemy magic user plane shift him away so he is out of the fight. Swamp him with smaller enemies while a stronger foe hints down other members of the party. I accept that my parties tank will probably be the last PC to die, as do they, it’s why he is always tooled up with healing potions, he is usually running around shoving them down unconscious players throats trying to keep them alive. 

Also, can he fly? If not put enemies out of range of him, a dragon that refuses to ground itself, harpies etc, anything that hit and runs the party. Or make the terrain work against them, a swamp where all moment is halved against creature that don’t have this issue, entangle him in a magical forest that you need to be delicate to move through. 

If you get the party thinking that they need to keep everyone alive not just the barbarian you start to make them think about buffing each other and spreading out support. If your players are getting one dimensional with their approach to combat encounters then change the encounters in a way they can’t anticipate. I find I do this not just for tanks but casters as well, wizard is sticking to the same load out, I put things in his way that make him think more about his spells etc. 

I think this works for anyone who has difficulties with overpowered players: deny their rest. Make them endure long journeys without a safe rest and they will be exhausted and out of abilities and spells (you can also throw levels of exhaustion). If they use Tiny Hut to rest a simple Dispel Magic from an enemy terminates the protection.

Quote from DMThac0 >>

My first answer is: Quit Giving Yourself A Handicap

Most DMs want to tell some epic saga of glory for our players to enjoy and participate in. As you have noticed with your group, not all players want to listen to the story. If your players want to fight their way from point A to point B with the story being in the background, then that's the style of game you have to run. If you insist on being a story telling DM with a table full of gamist players, you will not have the experience you want as DM. If they are going to be a gamist group, be a gamist DM. Toss in the story elements when it's necessary to move the plot forward, just like a game. If you aren't willing to make that compromise, then you'll be at a handicap the entire game and it may be better to wrap up the game and find a group that fits your style. 

Not all fights are fair, and that's good.

The rest of your party doesn't share the same type of survival tools as the Barbarian which results in a glaring issue when it comes to combat. So they have compensated by letting the Barbarian do the heavy lifting and they assist, even if they do wade into the fray, they know the Barbarian is their ace. It's a gamist strategy, it's an effective strategy, and it's very appealing since it feels like an "I win" button. However this also puts them in a bad spot since they're reliant on this, and that is where the party will fall if it's taken advantage of. It may seem unfair to use the Barbarian's ability to tank everything against the party, but not every enemy is that narrow minded. 

Have the enemies target the weaker party members. Take them out one by one, disable them, and otherwise remove them from the fray. At this point in time, at level 12, they are becoming household names as heroes. This means the bad guys are going to recognize them, know about them, and they should have information about the party and what they're capable of. This means their strategies will change to fit the scenarios and they will exploit weaknesses. Spells, weapons, terrain, and locations of battles will be chosen, at least by the intelligent enemies, to reduce the risk of defeat based on what they know about the party. Bottle necks, pits, guerrilla tactics, height advantage, traps, and more can be used to great effect in reducing the party's strength in combat, not just the creature's stat blocks. Even a single, well placed, Wall of Force can completely negate the Barbarian in combat while he's forced to watch the rest of the party struggle. Fair, no, effective, yes, and it is definitely something an evil villain would do if they had knowledge of their opponents.

I'm the DM so shut up...

This right here is also your "I win" button and can be used to great effect. Stat blocks, enemy abilities, attacks, damages, every little detail in the books is at the whim of you, the DM. You have the ability to create monsters that are a challenge to the party because they're not in the books. You want to mess with them and give them a Troll/Ochre Ooze hybrid that has acid immunity and can split when hit by slashing or lightning damage, go for it. You want to create a weapon for the enemy that pacifies aggressive creatures, make it happen. You have the freedom to create anything an everything, and that is where you have the ability to make the game challenging again. Get out of the books and get into the minds of your players, what can you craft that will really put them at the edge of their seats. Trope and rote ideas, concepts, tricks, and tactics may feel old hat to you, but they are great foundations for crafting the unique.

 

"This means the bad guys are going to recognize them, know about them, and they should have information about the party and what they're capable of. This means their strategies will change to fit the scenarios and they will exploit weaknesses."

Love this idea...makes sense and can be fit in the story nicely

This is beautiful! 

Intellect devourers and mind flayers

Bro just throw mind flayers at him and watch him kill his friends, easy

That day the Barbarian becomes way far stronger than the rest of the party, it's the right time to improve the menacing encounters over that party.
A way to improve is:

       *  How many ""flaws"" have the barbarian ??? did you take a look at his/her charactersheet ???

       * Do you know how many traps can you place in a dungeon ???

Wights don't do this any longer & the stupid Bear Barbarian even gets resistance to it.

You know because the Barbarian is akin to the Spirit of a Bear... An Animal that in 5E gets No resistances, but can give 20 or so to a mountain man that gets pouty and mad.

Doesn't that make so much sense? 

I ran a 5E game that was going fairly well until 2 Barbarians joined the group. 1 was Bear Totem. And then I realized how ridiculous this Class was. If you look back at the last 2 Editions, Rage had limitations & downsides. And it didn't last nearly as long. I'm betting that in their quest to create this Hyper Player Centric Faux Video Game, WOTC decided to remove all limitations during the final editing phase. Which grotesquely threw it further out of balance. They weren't the only Class either.

Bear Totem doesn't even make any sense, the average 1st Level Barbarian could take apart a Bear.

A Bear....who in 5E has No resistances, but a Barbarian who thinks he is 1 gets to effectively double his HP. 

Long story short, I quickly figured out how to combat these annoying characters. Simply award the Monsters with Player Levels. Suddenly the PCs get to see how stupidly overpowered their abilities are. Seems to me a Bear could certainly get mad and Rage. And they naturally can have Bear Totem.

To throw my thoughts into the pot:

1: The player picked out a bear totem barbarian because they wanted to be able to tank hits and beat stuff down in melee. If you start fudging rolls to make them take more damage then you're invalidating that feature, which they chose. As such, I recommend against this sort of thing, because it makes their decision mean nothing because you are changing the world to undo the results.

2: Barbarians are melee tanks, and as such, you will find them to be very effective when asked to tank hits in melee. Pit them against a flying enemy, a ranged enemy, or even goblins up trees. Start the combat with the opponents loosing shots at longbow maximum range at the party - does the barbarian want to rage now, and probably run out before they make it to the longbowmen?

3: Rage has limited uses, so make them choose. If you do 2-3 combats per long rest, they will be using a rage every combat. Here are some ideas:

• More encounters! If your barbarian has 4 rages to use, give the party 5-6 encounters per long rest. Don't let rage be a given!
• Trick encounters. If your barbarian starts every combat at round 1 by saying "I Rage!", then throw some illusions and trickery their way. "A huge shadow emerges down the hallway, and (rolls) throws a ball of fire past your head, what do you do?" - "I rage, and attack!" - "your attack hits, and the trenchcoated figure falls backwards, and a dying kobold with stilts on his feet rolls out, a spent scroll in his hand.". One trivial encounter and a wasted rage.
• Multiple enemies, from multiple fronts. A barbarian can't be everywhere at once, so giving the party 2 enemies to face from two sides will split them without splitting them - they can't all pour buffs on the barbarian if there's a second giant flaming scorpon approaching from the other side of them - the barbarian might solo one of them whilst the party deals with the other, but then the barbarian can expect to be more hurt than the others. To really get them, have the second enemy be larger, and wait for the barbarian to commit before revealing them, so the rest of the party has a bigger fight on their hands than the barbarian.
• More, smaller enemies. Barbarians hit hard but do poorly against hordes.
• Range! A barbarian might find their match when faced with half a dozen goblins at the top of some trees, with shortbows.
• Magic - get the barbarian hit with some fun spells to make the party cope without them (note: don't always pick on the barbarian, make sure everyone gets a far portion of this sort of thing). A faerie dragon polymorphing them into a badger, for example. They can still be an angry badger, but they aren't a greatsword-swinging half-orc any more!
• Draw out the combats. You could have them face a dragon and slay it over 4 rounds, but instead have the dragon disengage and move away, leaving the barbarians rage to simmer down. Having a big fight last over several encounters is a good way to avoid the players just using all their abilities, especially timed ones like rage!
• Sneak attacks. Geting the jump on them could mean 2 rounds of pelting rocks at the biggest guy there (the barbarian), whilst he's not had a chance to rage yet.
• Small ambushes: Barbarians get the chance to avod being surprised if they rage at a certain level, so if you are at that stage, throw ambushes which makes them decide whether to rage or not. Don't reveal the size, and increase on the fly if you need to.
• Traps in combat: An ambush starting when the barbarian is swooped into the trees in a net is going to be a fun one, and it'll keep the party from resorting to "everyone buff the barbarian" tactics for everything!