What genre are you writing for? Is it medieval fantasy? Dark fantasy? Cyberpunk? Where does that clan come from? Does it speak Common language or their own dialect? Thinking about other details helps a lot in the difficult naming process. I, for one, never name any character or group until thinking very hard about who they are, what they do, where they come from. I could just give you random names like "Ankkath", "Rangkor", "The Impalers" or "Bone Gnawers", but frankly, without more detail it is really really hard.
1) Take a random collection of vowels and consonants, preferably not currently in use by any English word. Gimme a second... Temoroth. 2) Take a modifier from their characteristics, i.e. how they do battle, the terrain where they live, their customs, relation to the plot. Let's say they like ambushes. The Shades of Temoroth 3) ???? (looting and pillaging) 4) Profit!
How about, "Those Guys.' But seriously, look at your story. How indepth have you made the culture of the people? Do they have their own language and words? Would any of those words be a good description for those evil barbarians? Alternately, do some research on cultures or people that were considered barbarians. You could name your evil barbarians 'Munchkins' if you wanted and it would sound appropriately terrifying if you explained it the proper way.
What I have found helpful in the past is to take words from that specific time period, write them out backwards and see if they can be pronounced. I played Pathfinder and Dungeons and Dragons a few years ago and I named all of my characters this way. Hope this helps, Good Luck.
Anything that sounds violent and vaguely medieval. Batman Begins got away with "Raas al Ghu" in a modern day setting.
How about "The Armageddon Sausages of Doom"? Now that's some intimidating shit. In all seriousness though, it wall depends on the setting. You could name them after the town/city/backdoor cesspool where they operate. "The Fists of Guacamole" for example, or "The Stormbringers of Manchester."
I have never thought much about taking up time deciding on "cool" names for things... If you have a name that works, then that's all you need. As long as it's a name that makes sense, perhaps in the context of the culture, and is something pronounceable, then you can let the storytelling do the rest. You can make the barbarian tribe sound scary by giving them an intimidating back-story or by having others react to hearing the name with fear and dread.
1.)Google some ancient Germanic names (I'm sure you'll find a few). 2.)Either take one of the names outright or alter it, and then name a clan after it. 3.)????? 4.)Profit.
I got some quick ones! Hammermen Skullbasher's Bonesmith's Facedeformers The Bludgeoning Band of Brutes Herecy's Joymen Hateful Sons Surgical Mishaper's Sinful Orchestra Krimson Crust The Happy Band of Violence Deadly Angry Masters of the North (DAMN) (Damnit! The DAMN's are comming!) ...uuhhh... Mindsplatters ... Silky Tocuher's ... Boneshatterers .. Brutefull Trust Mindfull Slaughterers Gruesome Joylings ... Naa I'm out...
I think of skulls and just about anything related to bone or bone tools when I picture and evil barbarian clan. So call them The Boner Clan.