Baekjo Mudeom

Anna, a dimensional traveler, enters the mansion of Margrave Lohengrin, known ominously as the "Swan Grave," as a maid in search of a way back to her original world. There, she encounters Margrave Rothbart, who mistakes Anna for his missing wife and forcibly takes her. He then makes her an unexpected offer:

"Be my wife in her place."

He then reveals that to return to her original world, Anna must "bear a child with a person from this world." As she ponders this, Rothbart's son, Svanhild, lingers by her side, pleading, "I want a mother, Anna." A relationship with a definite end. A condition she cannot refuse. Desperate to return to her original world, Anna accepts Rothbart's offer...

(Source: Ridibooks, translated)