This can only be obtained DURING a level 20 cap event that is in conjunction with the Pirates Chart. It has the same effect on the NMs as Animas do on ENMs. And yes it is fished up but you must be in the event to fish it up. I dont know if you can use them outside of the event but I am going to try to use one on the Sea Horror.
Really it is fished up and used during the Pirate's Chart battle.
Edit: Ok let me add more info the battle is explained here: http://ffxi.allakhazam.com/db/jobs.html?fjob=10;mid=1131809010154689476;num=4 this one weakens them by stopping hundred fists