The Fremennik blade is dropped by Fremennik guards, Fremennik warriors, Lanzig, Koschei the Deathless, and Dagannoths on Waterbirth Island (including Dagannoth Kings). Although the sword is not tradeable, it is possible to cast High Level Alchemy on it to obtain 3000 coins, or Low Alchemy to obtain 2000 coins.
30 Attack and completion of The Fremennik Trials quest are required to wield the blade. It has the same stats as an adamant scimitar, but with the speed of a longsword, thus making it inferior to the adamant scimitar.
Bardur, located in the Waterbirth Island dungeon, will trade you cooked Sharks for Fremennik blades, Fremennik shields or Fremennik helms.