Daru Secret Orders is the item you always needed to complete your quest. It's the item that starts your quest and if it would not be removed then you would be able to start and complete the quest again and again. You need to keep it in your inventory or you will not be able to complete the quest.
Also, pay attention, that quest item is temporary and it will disappear in 24 hours, you can't save those for later on and do many at a time.