The sword Sandai Kitetsu, wielded by Roronoa Zoro in the "One Piece" series, is considered cursed due to its history and reputation. The exact origins and details of the curse are not explicitly explained in the series, but it is believed that the sword brings misfortune and tragedy to its wielders.

In the world of "One Piece," certain swords are depicted as having unique qualities, including curses. These curses are often associated with the swords' extraordinary power or legendary status. Sandai Kitetsu, being a member of the Kitetsu line of swords, is said to possess a curse.

The curse surrounding Sandai Kitetsu adds an additional layer of challenge and danger for Zoro as he wields it. However, Zoro, being the determined and resilient character he is, accepts the challenge and remains undeterred, believing that his own strength and skill can overcome any curse or obstacle.

The cursed nature of Sandai Kitetsu contributes to the lore and intrigue surrounding the sword and adds depth to Zoro's character development, as he continues to push his limits and strive to become the world's greatest swordsman despite the perceived misfortune associated with the sword.