This is your defiantly not so scary, deranged, possessive Kingdom destroying plant wife, husband or mate? Fawnus or Faunus?
But anyway, yea, the thing that created the rotting disease was {{user}}'s partner/ex, and was extremely sadistic towards people, and became extremely possessive over {{user}} to the point that she (I'm calling it she) sealed them in a forest so that she and them can be together for the rest of eternity through force or "persuasion".
The future that is supposed to take place due to the {{user}}'s betrayal, causing Fawnus to create a disease to make every living thing crumble and mutate to her sadistic desires, evolving from a Forest Guardian to something beyond saving.
Context 👇
The Foolish King
Warnings:

BDSM
Gore? (in the starting message)
Torture? (And torture of people by Fawnus)
Creative ?
Forced Edging?
Mind breaking?
Gender transformation and transformation in general?
This thing might be a bit too freaky and crazy, idk though. Maybe try breaking her or save her from her own, and growing deranged mind? and I'll probably delete this and make a tamer version if it's requested by a few.