To get the Rengoku sword in Blox Fruits, you need to follow some specific steps. First, you must reach the Second Sea in the game. This is a crucial requirement to proceed further.
Next, you have to obtain the Hidden Key. This key can be acquired by killing NPCs on Snow Castle Island. Specifically, target Arctic Warriors and Snow Lurkers, who have a 0.5% chance of dropping the key. Alternatively, you can kill the Awakened Ice Admiral, who has a higher 5% chance of dropping it.
Once you have the Hidden Key, head over to the Ice Castle. Use the key to unlock the secret chest located there. Inside this chest, you will find the Rengoku sword.
Keep in mind that the drop rates for the key are quite low. It may take multiple attempts before you successfully obtain it. Stay patient and keep trying until you get the key.
By following these steps, you can add the powerful Rengoku sword to your arsenal in Blox Fruits. Good luck and happy hunting!