Some time ago the Beast was my PC for the Serpent's Skull adventure path. He was a stupid, stinky killing machine so they party kept him around. After all when you are an adventurer having an effective killer on your side is always a benefit.
Alas, in one encounter with a Hag he failed two consecutive will saves and was slain. He was replaced with my attempt at a combat rogue (who proved to be disappointing from a combat perspective) and I've never played "the Beast" again. He only made it to level 3 so I never really got a good fell for the build and I'd love to give it another try.
He joined an NPC adventuring party that went against the PCs in Book 2 of Way of the Wicked. He traded blow for blow with Gorbatch the Bloodrager but apparently Displacement is greater than a few points of Damage Resistance since Gorbatch proved to be the winner of that contest.
The build is constantly being tweaked as I look at adding in new things or subbing out stuff I'm no longer crazy about. The original Beast was very Min/Maxed with two big dump stats and I didn't really like that. Playing as a Human with Dual Talent means that I could boost Str and Con and not dump Int and Cha into the ground. The new version was slightly less effective in combat but a bit more well rounded PC which seemed a fair trade.
The basics of the Beast are: Invulnerable Rager archetype - grants DR = half your level and a bit of Fire or Cold resistance. The beast Totem line of Rage powers for Pounce and a natural armor bonus. Superstition for crazy high saving throws, Witch Hunter for bonus damage and and Spell Sunder because depending how you read it, you can suppress any spell effect. Reckless Abandon to counter act your Power attack penalties. Oh course the cherry on the cake is the Come and Get Me rage power you can take at level 12.
The latest thing that I've been looking at are the two Stalwart feats. The best way to cover the Feat prerequisites is to take one level of the Unbreakable Fighter archetype. The Stalwart feats change your Dodge bonus when fighting defensively or using Combat Expertise into DR that stacks with regular barbarian DR. Their seem to be two ways to go about doing this.
The first is Combat Expertise. It has the major downside or giving you an ugly penalty to attack which gets bigger as you level even though you maxed out your DR at 10 with Improved Stalwart. Combat Expertise has a prerequisite of an Int score of 13+ - which would normally be a dump stat.
Option two is fighting defensively. Getting a big enough dodge bonus to make this worth while means maxing out acrobatics and finding some other source like the pre-nerf Crane style feats. Pre-nerf Crane style is pure awesome sauce but comes with some complications. It has some steep feat prerequisites on a build that is already pretty tight on feats. So of course the best way to get around this to take two levels of Monk (Master of Many Styles archetype). What? We have conflicting alignment problems? Well I guess that's not going to work.
So there you go. You can go with the less optimal Combat expertise, try to find some way to get the 5 extra feats required to get Crane style (this might be viable in a Mythic game), or do some weird shenanigans to get levels in both Monk and Barbarian. Even this isn't ideal though since that's three non Barbarian levels which is a noticeable delay on the nice Rage powers. That and Crane Style limits you to a one handed weapon (although it can still be gripped with two hands on your turn).
So is Stalwart worth all of the effort? Is stacking DR 10/- really worth the trouble? At this point I'm still on the fence. DR 20/- at level 20 would be pretty great and the barbarian isn't really hurting in the "To Hit" so I'm inclined to say "yes", I'd just like a cleaner way to pull it off.
A few other things are worthy of special mention. In a game where you can really customize your gear there are two weapon enhancements that are no-brainers for a barbarian. Furious is the first. For the cost of +1 it adds +2 to your weapon when you are raging (which is pretty much always). Courageous is the second. This one is a bit more complicated. It adds half of the enhancement bonus of your weapon to ALL of your current Morale bonuses. Wait a minute, you say. The barbarian has tons of morale bonuses. The Strength and Constitution bonuses are both Morale. Will save bonus? Morale. The Superstition bonus is Morale. Got Heroism? Morale. Good Hope is a Morale bonus too. A Courageous Furious +5 weapon adds +3 to all of the bonuses. That's a lot!
The Beast is a walking mass of melee destruction. Easy to hit but with high DR and a metric crapton of HP it doesn't matter so much although you may need a cleric on standby to clean up after any really long battles. Expect to get left behind when the wizard does an emergency evacuation due to you resisting the spell because of superstition. The Beast will have to be strategic about turning off Rage.