Aku no Meshitsukai

A prophecy says that a child born with a mark on his back will bring an end to the kingdom, and that his death will destroy it. When twins Len and Rin are born, Len bears the mark and is banished to a lonely tower, while Rin is raised as the kingdom’s only princess. The siblings cannot be kept apart for long, and they grow deeply attached to one another. Though circumstances threaten to pull them apart, their bond endures as they struggle together to survive the dangerous world they were born into.