This Seeking Tech guide will explain how you can find the Blood Sword in Final Fantasy II Pixel Remaster.
You only have a limited time frame when it comes to finding this weapon. After beating the Mysidian Tower, return to Castle Fynn. From there, talk to Hilda and learn the Cyclone keyword.
Afterwards, head straight to Paul’s home in Fynn and then ask him about the Cyclone. He will tell you about a secret passageway near his bed. This hidden room contains 8 treasure chests, including one that contains the Blood Sword.