アベルと魔界の王

Abel to Makai no Ou

Despite being deeply trusted by the citizens, Prince Abel, the second in line to the throne, is shunned by his father and older brother, the first prince, due to a peculiar birthmark on his neck. When his arrogant brother displays resentment towards becoming the next king, a humanoid demon appears, offering to grant any wish in exchange for assistance. (Source: Natalie, translated)