"El caso extraño del Doctor Jekyll" by Robert Louis Stevenson is a Gothic horror novella published in 1886. The story follows lawyer Gabriel John Utterson as he investigates the mysterious connection between his old friend, Dr. Henry Jekyll, and the sinister Edward Hyde. At its core, the work explores the duality of human nature—the coexistence of good and evil within a single person. Jekyll creates a potion that separates these opposing forces, transforming him into the monstrous Hyde. The novella became an immediate bestseller and continues to inspire adaptations worldwide.