Dangsinege Bachineun Seonmul

A hero who sets out to the Demon World in search of the 'Demon King's Sword' to prevent the absurd engagement of his beloved princess. The Demon King desires her when he sees her, but the hero, who has sworn allegiance to the royal family, does not succumb to the Demon King's temptation. Then, the Demon King appears before her in the form of the princess, the being she loves the most. In the end, the hero, who falls for the illusion, is imprisoned deep in the Demon World's castle, believing the Demon King to be the princess, and living her entire life trapped in a sweet illusion.

(Source: Ridibooks, translated)

Note: Part of Ridi's Jujonggwangye GL Danpyeonseon (주종관계 GL 단편선) Collection.