The Lake of Rot gets its name for its red water, which will give a player scarlet rot if you venture into it for too long. Again, while it’s not a necessary area in the game, it’s still another piece of the map that you’ll want to explore to get the most out of your playthrough in Elden Ring.

The Lake of Rot is in a pretty secluded area of the map, and there’s really only one reliable way to access it in the game.

Source: https://www.distractify.com/p/where-is-lake-of-rot-elden-ring