It’s a cosmetic for the Leviathan’s Fang Rod which you acquire by taking down the Scylla Boss once you reach the very depths of the Calm Zone. Also read: How to get Angler’s Lantern in Fisch ...

Next, you must also first catch the Secret rarity fish from the three previous regions to unlock the Calm Zone bestiary. Hence, you must have the Magma Leviathan (Volcanic Vents), Frozen Leviathan ...