Death IV opens with the return of the youngest of the five siblings, the Blue Sister. She returns to find her country in a state of unrest as the massive armies of the Vergebian Union approach. 

She resolves to help her eldest brother the First Born as he and the Second Sun prepare to defend their House of Stone.
Meanwhile, the Black Sister and Little Brother conceive of a plan to infiltrate the Vergebian Union's camps and kill their General, Vagus.
As the Vergebian Union arrives at the rim of the Northern mountain ranges that protect the House of Stone, the call to arms is sounded and the entire country prepares to fight for their lives.