Kurenai no Shinigami wa Nemurihime no Neoki ni Nayamasareru  

Princess Lilith of the Kingdom of Amaryllis is to have a political marriage for an alliance with the Empire of Ayrras. Her husband-to-be is Crown Prince Gesualdo, who is feared as the "Crimson Reaper." Gesualdo does not trust Lilith and maintains a cold attitude towards her. Not discouraged, Lilith has no intention of remaining a decorative princess and boldly confronts Gesualdo. However, Lilith actually has a secret...!
(Source: Shufu to Seikatsusha, translated)
Note: Includes 3 extra chapters.