When Lord of Hatred launched, one unexpected feature that was added was Fishing. At the time, Blizzard also released a MeFISHto Infomercial teasing the Lord of Relaxation.
In Season 15, the way to spawn and defeat Mefishto has been discovered, thanks to the Diablo 4 Secret Finding Discord: Not Finding A Cow Level.
Fishing Up Mefishto, the Lord of Relaxation.
First, you will need to fish up a total of 10 Legendary or Unique fish, to combine them in the Horadric Cube. You can combine different types of fish as long as they are all Legendary or Unique.
This will give you a Torn Net, and a system message will say, “You emit a foul, hateful stench.”
Next, you will need to go to a hostile area, outside of Towns, where you can fight enemies and try to fish up Mefishto. Fishing him up could take a while, with some players reporting that it took them over an hour of fishing.
When defeated Mefishto can drop the Godly Plate of the Whale, which is a Chest Armor with the Unique Power: “Increases Fishing yelds by 100%. Does not apply to enemies caught by fishing.”