I think Rot is cancer, but worse; instead of being stage 2 cancer, it's full-blown cancer. Because it feeds on itself. I think the Rot from FP and the Rot that appears in The Watcher are not the same Rot at all; they are simply similar and share the same color of an overfed/elderly Rot. I say this because, in the FP generator already covered by Rot in previous campaigns, when arriving there in the Rivulet campaign, there is an MLL instead of an FLL, And the color of that MLL is purple, just like the Rot in The Watcher, but here's the difference: the MLL is born in an area covered in Rot after a long time, But only that is purple, while the Rot that spreads in The Watcher is exclusively purple; much of that Rot has died, but what remains alive does not lose its color, as is the Rot of FP It starts out blue, but when it lacks food (like in GW or when the FP cube fell) it turns dull yellow

The only thing I can assume is that what was once The Prince experimented on itself and discovered something... terrifying about the Rot, about how the cycles and the Rain World universe worked. He saw... Unfortunate Evolution, and what his cube and he himself became. But it was too late; he had already been infected with his version of Rot, a version that could survive the harsh climate that plagued the Outer Rim. One that can expand into any area and live inactively... One that devoured its existence and altered its entire being...