The Bandit Clans
The Bandit Clans will attacks player’s cities in every opportunity. And players attack them for loot.
These NPC Bandit Clans are there for players to attack and get extra resources quiet easily. The Bandit Clans will level up each time you destroy their City Center, they also become increasingly more difficult with each level. They max level is 40 and when you reach level 40, these clans will receive a 4 hour protection each time you destroy their City Centers. This was added to reduce NPC farming and promote PvP combat.
Demon King Clan
Having moved from one dangerous place to another all their lives, these paranoid nomads have mastered the art of misdirection. Beware the deadly maze they build in their settlements!
Iron Hand Clan
These ruthless savages may not look so smart, but they will surely look impressive before they crush you with a single swing of their fist!
Raging Bull Clan
Empowered with black magic and power of the earthbound spirits, these savages exert wrath on any who stand in their way.
Shrieking Dragon Clan
These merciless barbarians, far from modern civilization and conventional technology, will destroy town after town just for some laughs and kicks.
The Bandit Clans are found on the Campaign Map here you can also see what level their are by looking at the dots, the further from the center the higher level the clan is.
Update
In the 2014-06-18 update Bandit Clans will no longer randomly attack player's cities.
Source: http://forum.peakgames.net/showthread.php?18400-Update-Preview-Next-week-maintenance-and-update