"Youjo Senki," also known as "The Saga of Tanya the Evil," is a Japanese light novel series written by Carlo Zen. It follows the story of a salaryman who is reincarnated as a young girl named Tanya Degurechaff in an alternate world resembling early 20th-century Europe, where magic exists and is used in warfare. Tanya, a ruthless and ambitious individual, seeks to climb the military ranks while battling against various nations. The series explores themes of war, power, and the consequences of one's actions, all while showcasing Tanya's strategic mind and her encounters with the mysterious being known as Being X.