Sword of Stallion: Taneuma to Yobareta Saikyou Kishi, Ringoku no Oujo wo Netore to Meijirareru

Rass Tarlion has fallen far. A mercenary who once single-handedly slayed a Greater Dragon, he now indulges in a hedonistic lifestyle, visiting brothels night after night, earning him the derisive nickname "The Stallion" by people all across the empire. Yet one day, the Imperial Princess Fiarca—his ex-fiancée who was thought to be dead—appears before him. To save the country from impending doom, the princess takes Rass back to the Imperial Palace...and tells him he has to seduce the crown prince's fiancée, Princess Tishna from the neighboring kingdom!

(Source: Yen Press)