A god in itself in a way.

Nasharr(2) Described her rot specificaly as " i am the rot that clings to the dead things that brings the dead back to the earth.... It isnt just mushrooms and fungi. That recycle things. all things are born, they live and they die. but not all fungi look the same, especially when you dont know what you are looking for, or when it is not what you want to see. " (Session 85)

Rot seems to mostly be controlled by the Lord Nash'gsteuf.

People can be Transformed by rot by going to Thynnyne, where they will meet sages Xaltr, Mulruch, Akhrin, Hykkrin (Deceased). It is always humans that are.