Warens are the locations in which weilders are trained. This is where they are molded into, what many would say, the best warriors in the world. Here they hone their mind, train their bodies, and understand their spirit. Here they become warriors.
Somehow, someway, a band of gnomes learns how to be ninjas. In fine gnomish style, they open an academy to teach this art to other gnomes. Wacky Hijinks ensue. Or the gnomes become some of the finest assassins in the land. Or both.