Witherbloom

Witherbloom is a Secret-rarity fish from Toxic Grove, and the wiki describes it as a withered variant of Rotbloom infused with ancient blight curses, too eerie to release. It is tagged as a Hunt fish. To catch it you need Night, Rain and Autumn all at once, and no bait is listed.

It is enormous: 55,000 kg minimum, 65,000 kg on average and up to 157,500 kg, ranking 39th of 663 by max weight. Because the rate per kg is only C$0.707, an average catch still sells for about C$45,933, 32nd of the 742 documented fish by average value. Base XP is 55,000, the kind of number that shifts your level on its own.

Blight Idol is the other Secret fish documented in the same zone, and the zone sells Decayed Rod and Plaguereaver, the natural picks for a Secret hunt. If you have the patience to wait for an Autumn rainy night, Witherbloom is one of the best payouts you can find at that tier. It is not casual fishing, but the sale price and XP justify a planned attempt.