Kouguu no Omega

Ilya, the 13th prince of a small kingdom, was born an unexpected—and very much unwanted—omega whose birth brought shame upon his royal family. As the lowest-ranking member and an omega, he is destined for a political marriage meant to improve his kingdom's standing. Resigned to his fate, Prince Ilya travels to his new faraway home. Upon entering the king's gilded palace, where he's expecting to meet his future husband, he instead finds a mysterious young boy with an unsettling message. (Source: SuBLime)