Spoiler alert: This information is extremely new and can spoil the story for readers who haven't read the story till its last update. Proceed at your own risk.
Not much is known about Feldeaths, aside from there being a moderate chance of encountering them in the Undercrust.
A Feldeath is created, according to Garovel, when a large group of people, such as an entire town, are killed and no reaper is present to escort them. The souls eventually coalesce into a whole being driven mad by the collective raw agony of being trapped on the mortal plane.
Only a small handful have ever been formed on the surface, due to the historically slim chance of a town being obliterated and forgotten. In the Undercrust however, there are far more, due to cave ins.
It ois also known that they have a conection to Sermung.