In Ancient Times, The Powerful Seize Step by Step

In the third year of wearing this ancient guise, Zhu Qingyu found her protector imprisoned. Gu Zhao, heir to the Duke of Dingguo, extended a helping hand. His condition was simple: "Once."

A single night of fleeting passion. For her, it was an end. For him, it was merely the beginning.

...

Gu Zhao adhered to a path of self-restraint and integrity, a man of principle and upright conduct. Yet, only with a married woman did he find himself unable to suppress boundless greed, obsession, and infatuation. He knew full well her gentle affection and delicate attentions were born of necessity, all for the sake of another man. Still, he could not help but demand more with each step, clinging to the fantasy that within the feigned tenderness she offered him, there might have been—even for a moment—a shred of sincerity.

Note: Both leads are chaste.