"The Mysterious Stranger" is a novel by Mark Twain, though it was left unfinished at the time of his death in 1910 and published posthumously in various versions. It's one of Twain's darker and more philosophical works, exploring themes of morality, human nature, and the nature of existence.

The story is set in medieval Austria and follows the experiences of a young boy named August Feldner who encounters a mysterious stranger named Satan. This enigmatic figure, who goes by various names throughout the story, possesses supernatural powers and challenges August's perceptions of reality, morality, and the nature of good and evil.

Throughout the narrative, the mysterious stranger exposes the hypocrisy and cruelty of human society, while also demonstrating the arbitrary and capricious nature of fate. The story is marked by its bleak outlook on human existence and its exploration of existential themes, including the meaninglessness of life and the absence of divine justice.

"The Mysterious Stranger" is considered one of Twain's most ambitious and experimental works, though its unfinished nature has led to multiple versions and interpretations over the years. Despite its darkness, the novel offers a thought-provoking exploration of fundamental questions about the human condition and the nature of reality.