Reimei no Arcana: The Beginning

Reimei no Arcana: Hajimari no Toki
黎明のアルカナ 始まりの刻

Source

Dawn of the Arcana
Reimei no Arcana· 黎明のアルカナ
manga

Princess Nakaba of Senan is forced to marry Prince Caesar of the enemy country Belquat, tantamount to becoming a hostage. While Caesar is pleasing to the eye, he is also selfish and possessive, telling Nakaba outright: “You are my property.” With only her attendant Loki at her side, Nakaba must find a way to cope with her hostile surroundings, her fake marriage...and a mysterious power! (Source: VIZ Media) *Note: Includes 12 extra chapters.*

Alternative

The King's Beast
Ou no Kemono: Enpei no Arcana· 王の獣~掩蔽のアルカナ~
manga

In this kingdom, Ajin are considered no better than beasts. The men are sent to war, the women serve in brothels. Those who are lucky enough to live to old age spend their last years doing degrading, menial work. Boys who show signs of magical ability must serve the imperial family in the palace. Rangetsu's twin brother is sent to the palace to serve the Prince Tenyou, but he only lasts a short time before he is murdered. Rumors say his master was the one who aimed the blade, and Rangetsu decides to puts aside her femaleness in order seek revenge. After years of excelling on the battlefield, she has finally found her way to Prince Tenyou's side. But he isn't what she expected, and the webs of intrigue in the palace is more tangled and venomous than she imagined. Can Rangetsu find her vengeance before she too falls prey to palace plots, and will her inconvenient feelings for the Prince Tenyou be a hindrance or a help? (Source: VIZ Media)