|“||Taking place in an area called the Mold Threshold, you can see spores and infectious mold taking over the room completely.||”|
–Elise Favis, Game Informer 
The Threshold consists of a large sublevel under the Research Sector proper which is now abandoned and coated in plant-like and fungal-like growths, and emerged in the Oldest House in 2019, being a recent threat. This "Mold" takes over all living things within in range of influence, including humans - and the area is populated with various humans who have fallen under its sway and become Mold Hosts. This threshold is believed to be the source of the Mold, and the area is currently under study by Raya Underhill. Sometime before the Hiss invasion, a Ranger squad under the command of Ranger Captain Ramsay entered the Threshold and even set up a Control Point. However, they all succumbed to the effects of the Mold, though their Control Point remains functional once it is cleansed.
In the midst of the Hiss invasion, Jesse Faden entered the Mold Threshold to stop the threat, eventually discovering and defeating the source of the Mold, a monstrous entity called Mold-1 within an even deeper area of the Threshold known as the Pit.
The mold is described as the result of incompatible molecular structures from different dimensions interacting, with the dominant one overriding the inferior one. The mold may therefore simply be a form of matter native to an extremely alien dimension. The mold takes over all living things within its range of influence, including humans.