This subclass is themed around Barbarians who draw their power from some sort of mystical beast. The Unearthed Arcana article features a table that you can roll on to determine the source of your hirsutism and hygiene issues. (The first one says werewolf, so no doubt that particular option will see quite a lot of action – Wizards certainly know how to play to a crowd!)
Many of the features below revolve around your Constitution modifier, which is worth bearing in mind if you're thinking of walking the Path of the Beast. Each of the features below includes a summarised reference to the rules in the Unearthed Arcana article but it's worth downloading the full, free PDF from the Wizards D&D site, here.
Form of the Beast - 3rd Level
When you Rage, manifest a natural weapon: Bite (1d8 damage, regain HP), Claws (1d6 damage, attack twice), Tail (1d12 damage, 10 foot melee range)
The 3rd level ability for this path grants you the power to partially shape shift so that you can manifest a natural weapon (kind of like the Alter Self spell, but with less magic and more ouch!) Each time you Rage, you can choose to gain the option to bite, claw or tail strike, each of which comes with its own benefits.
Bite lets you make like a Lizardfolk and actually eat your enemies. This may sound cool, but remember that you sometimes have to fight Trolls, and there is no way that tastes good. You get to regain health if you do this, which is good for durability – particularly as you'll be getting Temporary Hit Points from later abilities, so this doesn't interfere with those.
Claws do less damage than bite, but they give you a chance to attack twice as one action. No bonus action involved here, people – that leaves you with your bonus action free to do whatever you want with. Admittedly, there isn't really much that the basic Barbarian tree allows you to do with your bonus action, but you can bet that there are some tasty combos out there for you to exploit if you multiclass. Slashing someone twice and then giving someone Inspiration as a Bard-bearian (I went there), just for starters!
Tail is the most powerful attack of the three options, and it has reach, making you a nightmare for annoyingly mobile characters expecting to be able to sneak past you.
Having the option to choose which type of natural weapon you want each time you Rage makes for a fantastically versatile feature – the role you are fulfilling within the party and the number and type of creatures you're facing off against will help you to decide which of your body parts you want to animalise in any given situation.
Bestial Soul - 6th Level
Form of the Beast natural weapons count as magical and choose one of: swim speed and water breathing, spider climb, or extended jump range.
What's better than natural weapons? Magical natural weapons! Get to level 6 and your Form of the Beast grants you just that. Magical piercing and slashing damage are not often resisted by your enemies, monstrous or otherwise, so this is a great way of ensuring you can keep pumping out damage – especially as you will have Extra Attack by the time you pick this up. (4 claw attacks per round, anyone?)
If that particular boon wasn't enough for you, you get the choice of either being able to swim and breathe underwater, spider-climb (eat your heart out, spellcasters) or extend your jumping range once each turn (by semi-random increments)... as a continuously active bonus. When you have a short or long rest, you can choose to switch to a different mode. Sometimes it seems like things are being printed just to trigger Ranger loyalists, and this very much feels like it falls into that category because being able to adapt to your environment like this is very useful indeed!
Infectious Fury - 10th Level
When you hit a creature with your natural weapon while raging, it must pass a Wis save or use its reaction to attack another creature or take damage.
Ever had rabies? Want to try it? OK, then!
This delightful little feature proves that your personal cleanliness leaves a lot to be desired. Each time you hit a target with your Form of the Beast weapons while you're raging, you force them to pass a saving throw (using your Con modifier as if it was your Spellcasting ability – I told you it was important!) or get cooties. These are the type of cooties that either force said target to perform a melee attack against another creature, or cause 2d12 psychic damage!
You're restricted to using this a number of times equal to your Con modifier per long rest (there's that stat again) but this is a big old “HEY PALADINS, HOLD MY BEER!”... except your smite dice have more sides.
Call the Hunt - 14th Level
A number of your allies gain the Reckless Attack feature when you Rage and you gain Temporary HP for each one that does.
Oh boy. This is now an epidemic.
Do your friends like having advantage on their melee attack rolls? Are they, by any chance, Rogues? Yes? You're now their top bae. Number one with a bullet. Even if they're not Rogues, they are probably considering sending you a Valentine's card.
The ability to give a number of friends equal to your Con (oh look, Con) modifier the Relentless Attack feature is good on its own, but do you know what's better? Letting them keep it for the duration of your Rage and getting 5 Temporary Hit Points for each one of them that accepts this blessing. And why wouldn't they? It gives them the feature, not the effects of the feature, which means they get to go ahead and choose if they want to activate it or do without gifting their enemies advantage on their attack rolls too.
That means that if you're the lone melee fighter in a party of weak and feeble spellcasters, you can just give them all this feature anyway, claim the hit points, and then literally not care whether they even so much as lift a melee weapon. You have your bonus health, that's what you came for, right?
Congratulations, you just became a legitimate beatstick. (Or was that beaststick...)
As a subclass this is really evocative, and the concept is likely to appeal to many players for various reasons. Some are going to like the idea of transforming during rages, and will be drawn to one of the natural weapon choices above the others. Others might like the idea of only one body part changing in some kind of grotesque semi-metamorphosis (paging the Simic Hybrids!). Either way, between the ability to deal awesome amounts of damage and incite frenzy in your whole party, this is one dangerous character choice that looks very effective, without any mechanical drawbacks like exhaustion to fall back on. The movement features add a strongly cinematic feature to the subclass, allowing you to gambol through open fields, skitter among the stalactites of dank caverns or, you know, take part in the Ghosts of Saltmarsh adventures without a care in the world...
A couple of things worth bearing in mind – while the bestial transformation that comes with your Rage in this subclass is always going to look incredibly cool, there is definitely room for your DM to decide that any innocent bystanders that see it happen will be forever more terrified of the sight of you, even in your normal form. Unless they're on Team Jacob, in which case you'll have a crowd of groupies to deal with. There's also nothing that states what type of beast you turn into – the choice is yours. This means that my lifelong dream of Were-crocodiles in 5e has now been achieved and I can be forever happy that my prayers to the dev team have at last been answered!