Abyssal Grenadier

Nights in the Passage of Oaths, part of the Cultist Lair, are when the Abyssal Grenadier bites. You need Fish Head on the hook, a Windy sky and winter on top of the night hours, so the window is narrow even for a Mythical fish.

Each catch is worth 2000 base XP, 73rd of the 100 Mythical fish and 408th out of 1,260 overall. Three fish match that figure exactly: Aurora Trout, Void Angler and Zeus' Herald. Wobbegong Shark follows lower at 1550, so the Grenadier sits in a crowded tie with 72 Mythical fish above it. Nothing in the listing gives a weight or a sell value for it, so XP is the only number that measures it.

The in-game line calls it a deep-sea rattail scavenging abyssal plains worldwide. The catch shouts are written in capitals: "AN ABYSSAL GRENADIER!", "RATTAIL TITAN!" and "CRUSHER FROM BELOW!". No other fish is listed as sharing its spot.