Flameslasher

The hardest catch of Fischfest 2 is Flameslasher. It lives in the Heart of the Island, a place that opens only after you place all 7 conches on the pedals, and reeling it in inflicts -100% progress speed. It bites at Night in Tropical Sun weather during Summer. It has a fixed 3% hook chance and can be caught only once, like the Limited Cryoshock Serpent, and landing it is required to finish Captain Conch's last quest. The wiki says the rod Captain Conch gives you is the one to use.

What makes it special is the fight. A heat bar appears under the progress bar, and flames home in on your control bar about every second. Each hit fills the heat bar, lowers control and raises progress speed by about 7% per flame; each miss drains the bar, returns control and lowers progress speed. When the bar nears full a warning shows, and when it is full it glows white, shakes and explodes, killing the player instantly.

Slash enchants fill the heat bar by a large amount when they activate, so they can cause that instant death; the restricted Sunshell Rod cannot trigger it.