It's available in single player, but not multiplayer. I don't think it's me. The mods I use are "Removable districts, slower tech by era, not so unique improvements, and more civs allowed (lets you have more then 12 players in multiplayer. I mean, I understand the limitation for human players, but for hot seat with a few humans, I like to have around 15 AI on a huge map.) But it wasn't available in multiplayer even before I got that mod.
This is a shame, because it really isn't worth running in single player. All it really does is give you the ability to buy cheap and sometimes unique units, which I have not seen the AI take advantage of. The developers literally gave zero thought to how the AI would handle this, dramatic ages, zombie defense, or even corporations.
This is hardly unfair in mutliplayer. What's really broken and unfair is Babylon earning a certain scientist who unlocks a random Eureka from the modern age, and then building a single Infantry or tank in the medieval era and wiping out entire empires with one unit.
The real reason there are no Barbarian Clans in multiplayer is because it's a scam, and people would catch on. You see, after the early game invasions, clans basically just sit there and do nothing for the rest of the game. They want the players to spend the ridiculous money for them to attack someone, and won't do it on their own anymore. Of course, this becomes a problem when there are multiple players, especially if everyone is a player.
While it's a fun idea on paper, in actual practice it isn't worth paying them. It probably isn't worth paying ransom either when you could buy a unit and get your builder / settler back with the gold instead. Why pay them to do something they would normally do for free, and what are you actually getting for your gold? By the time you can afford to pay them to attack someone, it's unlikely they could wipe out any cities. The most that will happen is a bit of light pillaging and maybe the loss of a unit or two, if you get really lucky they'll capture a builder or settler, but again, you're just paying them to do something they would normally do anyway.
