Ozark Cavefish

A translucent, blind cavefish from Ozark karst aquifers, says the in-game text, and Fisch places it in The Sanctum, the Cultist Lair. It is Legendary, yet its bait is only the Common Worm, so the hard part is the conditions: Night, Foggy weather and Autumn.

Its 1800 XP puts it 37th among 103 Legendary fish and 423rd of 1260, in a tie with Coral Emperor, Leopard Shark, Yeti Crab and Ginsu Shark. No other fish is listed for The Sanctum, so the cavefish has the spot to itself, and its quips are "AN OZARK CAVEFISH!", "TRANSLUCENT TREASURE!" and "VOID VETERAN!". The wiki adds no weight, price or mechanics note for it, which leaves the XP figure as the best way to measure it against other Legendary fish.