Sword of the Stallion, Vol. 1 (Light Novel) - Gakuto Mikumo

An honorable knight fallen from grace...returns as a total stud! Ras Talion was once a respectable knight who single-handedly slayed a Greater Dragon, but he lost his beloved in that same battle. Now Ras is a mercenary and a hedonist who visits brothels night after night. He's even known throughout the empire by the derisive nickname the Stallion. One day, he comes face-to-face with the impossible--the supposedly deceased imperial princess Fiarca, his ex-fiancée. Alive and kicking, she assigns him a mission she believes only he can accomplish. To save the empire from impending doom, he must seduce a princess of the neighboring kingdom!