The Argent Crusade is hardly the first force to challenge the Lich King. Our enemy turns spirits of the fallen against us.
Their spirits wander the Valley of Fallen Heroes, southwest of the grounds and between Aldur'thar and the Ironwall Rampart. The area is littered with discarded soul crystals, used by the Scourge to control the spirits. Gather those soul crystals and use them with this Light-blessed relic to bestow the blessing of peace upon the spirits. They have more than earned their rest.
You have done our fallen brethren a great service and spared us the need to face them again on the field of battle. As our campaign against the Scourge progresses, I find myself thinking about my own fate.
Should I fall, <name>, I would want someone like you to do for me what you have done for the spirits of the fallen this day. I could not bear the thought of forcing my own brothers to slay me again and again.