Western Xia Regime

Why is the dynasty that once dominated ancient China with Song and Liao States and once was among the three most powerful kingdoms together with Song and Jin States absent from historical records? Why did such a strong and powerful nation that stood firmly in the Northwest against Central Plain for several centuries melt away like the snow? Why did Genghis Khan completely eliminate the race and culture of Western Xia regime from the history? Western Xia Regime, a long historical novel, will give you all the answers.