In accordance with the post-a-day challenge by SunderedWorldDM, whose dreams bore the Orbseeker and whose deeds send the world shuddering.
Tribal societies are common in many fantasy settings, but are often the result of reflective depictions and stereotypes. It doesn't help that a book may simply refer to a particular society of humanoids as a tribe without any other note, save perhaps to name a chieftain.
The academic definition of a tribe is contested, sometimes hotly. For the purposes of this article, we'll say a tribe is a group of people with a common identity and social organization, but with few formal institutions or explict complex hierarchies. (Even this definition is foolishly constructed, and leaves out plenty of tribes of the world. Remember, the type specimen for "tribe" in English etymology is the Twelve Tribes of Israel.) One way to make fictional tribes interesting is to go out of your way to shirk one of the assumptions one might draw around tribal groups.
Assumption: tribes are technologically inferior
Assumption: tribes are nomadic
Assumption: tribes are mystically connected to the land
Assumption: tribes are rural
Assumption: tribes are traditionalist