On this page of our Elden Ring guide, you can view the locations of Ashes of War and Spirit Ashes found in Mohgwyn Palace, a region which is accessible during the course of White Mask Varre quest.
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.
Go to the area marked on your map - there is a body in the nearby cave. Simply pick up the ash from the body.