Ashes of War are special items that allow you to change the active skill of your weapon. Thanks to them, you can exchange your weapon skill for a completely new one. You activate them by pressing the button responsible for using skills. Spirit Ashes, on the other hand, are items that, when activated, summon spirit companions that will help you in combat.
To get this ash, you need to visit the location at night, as you need to face another Night's Cavalry. Simply wait until nighttime, travel to the marked location and defeat a side boss.