Blight Idol

Blight Idol is a Secret fish from Toxic Grove, and its price tag is the first thing worth noticing: C$56,000 per kg. It only weighs between 0.3 and 0.5 kg, averaging 0.4 kg, so the average sale lands at C$16,800. That is the 97th highest of the 742 documented fish, even though its maximum weight ranks 648th of 663. In other words, the value comes from the rate per kg, not from size.

To catch it you need Coal as bait and an Autumn setting; the wiki lists no time of day or weather limits. The base XP is 12,000.

In Toxic Grove the only other Secret fish is Witherbloom, so the top tier of the zone is a short list. Two rods are sold in the same area, Decayed Rod and Plaguereaver, which means you can buy gear without leaving.

Verdict: if you fish Toxic Grove, this is the one to chase. Bring Coal, wait for Autumn, and expect a small fish with a large payout. Selling it makes sense because a catch weighing less than half a kilogram still fetches thousands of C$.