Tantalum is a rare, blue-grey metal that is found deep within the chasms of Ferchiun*. Immune to all but the strongest acids, if more had heard of it would be more sought after. It is nearly as hard as adamantine and holds an edge equally as well. Those few who believed such a tale did not return from their quest to gather any, or if they did, it was not to the homes from which they came.
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.