Two Names of Night

Rose falls asleep at home and nearly faints when she wakes up at the palace in the arms of Emperor Lukewarell. Despite her uncanny resemblance to his beloved concubine, Agnes, Rose manages to convince Lukewarell that she is an ordinary tutor with no relation. However, she must help him find the missing Agnes by masquerading as the concubine and feeling out the suspicious people around her. But the more Rose learns about Agnes' complicated relationships, the less she understands who this mysterious woman was. In fact, the dread in her heart only grows as she gets closer to the truth...

(Source: Tappytoon)

Note: First released in English, with the initial eligible release in Japanese followed by Korean, making it a manga by our standards.