I was making a class and i decided to make a level 1 barbarian and add a second class to it so i chose to be a sorcerer as well with draconic ancestry and i realized i made a battle magic user that can kill you in more ways than 1 as i can rage, use spells, turn into a part dragon and have a F*@# ton of health!!
So my question is what is more scary with the barbarian?
The problem with multiclassing into a Spell Caster is you can't cast spells while you are Raging and you can't even Concentrate on spells while Raging. You're not focusing on one thing to be good at, spreading yourself to thin.
That said, in my personal opinion. 1-2 lvls of Barbarian and the rest of it Moon Druid is an amazing combination. Moon Druids care almost nothing for their spell slots and the Rage will add to Wild Shape's sustainability.
That said this is all very subjective, because every defines "scariness" differently. For my The Above character I made was very more effective incombat then I ever expected. I was worried I might outshine the party too much, but it balanced out after lvl 3 pretty quickly.
I imagine the scariest multiclass for a barb would be 9 totem barb/2 fighter/9 assassin rogue. You only really need three levels in assassin for the scariness, but you can grab more for extra damage and defensive features. For fighter, pick twf. For barb, pick up elk as your first totem for extra speed and wolf as your second totem for sneaking around more quickly. Wield two rapiers and pick dual wielder for one of your feats. Surprise your enemy and use your action surge for an extra action. Every single hit you make crits. For your first turn, charge up and make three weapon attacks (two from extra attack and one from twf). For your action surge, make make two weapon attacks with your extra attack. If the dude(s) is dead, run off to where people can't easily see you. If you have a cloak of invisibility and drank a potion of speed (add another attack), this is even scarier. If someone pulled this on my party, I would be shitting my pants.
If my math is right, you're doing 20d8 brutal crit with rapier damage + 10d6 sneak attack crit damage + 5x str mod damage + 15 rage damage in a single round if all your attacks hit. Yikes!
In terms of Mechanics its hard to beat the Moon Druid/Totem Barbarian.
But Zealot Barb/Spore Druid would function ok Mechanically, and the visual interactions between Divine Fury (necrotic) and Symbiotic Entity would pretty rad. And 1d6 Necrotic, + 1d6 Poison on hits would be pretty nice...
I like Barbarian / Bards. They can grapple better than pretty much anything else, do crazy stuff like grab someone, dimension door them hundreds of feet up and suplex them down to the earth, throw out inspiration all over even while raging, and you can contribute a ton in and out of combat. Decent at will damage, skill monkey, and it's pretty different, and not too often seen.
Although MAD, Barbarian/Paladin can be interesting, especially if you use spell slots for divine smites. Plus, there's some great scariness there.
"Foolish little man! You have angered my god. But even worse, you have angered ME!"
(For bonus points, take Zealot as your barbarian subclass.)
Barbarian/Rogue could be quite scary and less MAD, too (maybe use strength with a rapier or knife?).
Few of the merchants noticed as Gorth the half-orc thief crept past. And those who did kept their mouths shut. Having your things stolen was bad, but having your skull crushed was even worse.
Barbarian/Cleric would be similar to Barbarian/Paladin, with some differences: while you wouldn't get the divine smite feature, you wouldn't have to put points into charisma, and could instead put them into wisdom, which is useful for almost any character anyways.
Barbarian/Fighter is powerful, but doesn't really add much scary flavor-wise that you don't already have. You could maybe make your character an accomplished warrior, and a calm one, besides. But when he loses his temper? You'd better watch out.
As other people have said scary can be variable based on your group, and totem/moon is pretty cool for tanking but here are a couple of ideas I've had for multiclass barbarians:
Frenzy barbarian/moon druid halfling: concept and idea is he's a feral dog and wants to be an apex predator so lots of focus on tracking prey, catching it by surprise, and keeping it pinned while you eat it alive.
Totem barbarian/ circle of the shepherd druid: same original headspace as the first one but instead of being a predator she's a pack alpha so skills focused on helping the group and leading people, mechanically focused on buffs and if your group allows feats pick up inspiring leader and healer for doubling down on the buffs.
Valor Bard/ancestor Barbarian Dwarf: pretty cut and dry its Sabaton in D&D, sing songs about glorious war and charge into melee as the Power of METAL fills you with adrenaline and you carve your mark on history.
Barbarian/Monk: not sure what specific subclasses I would want but conceptually having someone who fights by either extreme self control and balance or absolute release of passion, I mostly just want to play one because previous editions made doing so impossible due to alignment restrictions. If anybody has a good idea/backstory for one I'd love to hear it though.
One of my first 5e groups ran through Princes Of The Apocalypse, and we had a guy playing a Hill Dwarf Bearbarian/Moon Druid, and it proved highly effective. I don't think he went above Barbarian 3, he wanted to get to Druid 10 for the Elemental Wild Shape forms. I think his two favorite beast forms were Giant Spider and later Giant Scorpion.
How about a ranger-like (not multiclassed with ranger :) ), With a multiclass with Wizard with divination? I have one and he never misses. Usually I say this on forums and people check my character after I say my Barbarian is ranged. They say I have no bow. I say I have two hand axes. :)