The Reincarnated Prince and Felvolk's Greatest Treasure (Volume 5) by Nobiru Kusunoki

Having defended itself against the invasion of the Atrad Empire and successfully eliminated the darkness choking the life out of the country, Gracis is now busy preparing for the upcoming harvest festival. Seven-year-old Prince Herscherik, the key figure responsible for bringing peace to the nation—and secretly the reincarnation of a thirty-four year old otaku spinster—now finds himself grappling with mountains of paperwork day in and day out. Desperate for a change of pace, Herscherik sets out on one of his usual incognito excursions into the castle town—but instead of a relaxing escape, he finds another mystery when he runs across a pair of shadowy strangers...