The Sarecans lived in the desert. They had no wood or concrete to build their buildings out of, so they used sand. All over, they buried food and treasure, intended to be dug up later. They were packrats, and their most valuable possesion was a map that could lead you through the desert to a new, uninhabbited land. This was thousands of years ago. A group of Sarecans memorized the map, and traveled to the new land. The became known today as peasents, evolving their own distinct looks and culture, different from the Sarecans. The skeletons are the ancient ancestors, the Sarecans who had originally traveled to the new land. They were killing the peasents because they were mad that the peasents abandoned the old culture. The necromancer it their savior, because he brought them back to life.
The Sarecans held on to the map, and had it for thousands of years, until those blasted knights took it by beating them at volleyball.