When the mod updates to 1.18, I believe that the Deepslate layer should generate in Middle Earth since it is just real life Slate, another layer of Stone. It could in the mod represent the deepest depths. There could even be some first age mobs hiding in the Deepslate area, devolved into an animal form. Perhaps Werewolf Dens and Vampire Nests could generate there to represent that other first age creatures besides Durin’s Bane went into hiding. However, if they are added, they should just be hostile on sight and not factions in order to heighten the effects that hiding for so long had on them, showing that the dangers of the depths don’t care about good vs evil. There could also be Void Fog that would generate down there as well as a new fog called Nameless Fog, which could represent the Nameless Things. This new fog would generate within Void Fog which would make it difficult to see anything while Nameless Fog would cause illusions and nausea to convey the sense of the mystery and powerlessness of the deepest depths. The horror of not knowing what might be just out of sight would be terrifying, but it does make sense to be added to represent the Nameless Things. The only thing that the Nameless Fog would not do would be dealing any damage to health.