Basilisk

For centuries, the Kouga and the Iga — rival ninja clans — have been trying to kill each other. A ceasefire finally brings peace, and the heirs of each clan — Gennosuke Kouga and Oboro Iga — fall in love, hoping their marriage will end the bloodshed for good. Then the shogunate decides to use their rivalry as a proxy war to determine political succession, and each clan picks ten of their deadliest warriors to fight to the death. Gennosuke and Oboro are on opposite sides of the kill list. This 24-episode TV series from 2005 is basically Romeo and Juliet with ninja death matches, set in 17th-century Japan. Every fighter has some bizarre, grotesque supernatural ability, and the show isn't shy about showing you what happens when those abilities collide. It's a seinen series through and through — dark, bloody, and emotionally heavy. The romance at the center isn't played for melodrama; it genuinely hurts to watch two people who love each other get crushed by forces way bigger than them. If you liked Ninja Scroll's unrelenting violence and strange powers, or the tragic weight of Rurouni Kenshin: Trust and Betrayal, this hits a similar nerve. Fans of Shinobi: Heart Under Blade will recognize the source material's DNA immediately. Studio Gonzo adapted the manga faithfully, and the pacing across 24 episodes keeps things tight — no filler arcs, just a countdown as the body count climbs. It's bleak, but it earns every bit of that bleakness.