That question instantly made me think of 'Vagabond' by Takehiko Inoue, even though it's a manga—it's the deepest exploration of the samurai psyche I've ever encountered. It's less about a simple revenge quest and more about the crushing weight of violence and the search for meaning beyond the sword. The revenge plot against the Yoshioka school is brutal, but the real intensity comes from the internal conflict Musashi faces. The art makes every swing of the blade feel visceral and consequential. It gets at something most novels miss: the sheer exhaustion and spiritual decay that comes from a life dedicated to vengeance, and whether honor can exist in that space.
For a more classic, unrelenting literary take, I'd point to 'Musashi' by Eiji Yoshikawa. It covers similar ground but feels more epic and historical. The revenge elements are woven into a larger tapestry of duels and personal growth, though the drive to prove oneself and destroy rivals is relentless. The famous duel on Ganryu Island is a masterclass in tension. It's a slower burn than some modern dark fantasies, but the darkness is in the societal constraints and the single-minded pursuit of being the best, which itself is a form of revenge against a world that initially saw him as a brute.
If you want something that blends the dark samurai ethos with almost supernatural horror, 'The Samurai' by Shuhei Fujisawa, which the film 'The Twilight Samurai' was based on, is incredibly bleak. It’s not a high-action revenge saga, but the revenge is social and quiet—a low-ranking samurai seeking justice and dignity in a corrupt system that's actively crushing him. The darkness is in the claustrophobic atmosphere and the inevitable, tragic cost. It feels more real and, in a way, more devastating than any fantastical bloodbath.