Nightmare.Jar is what the Cave Dweller could've been, it lurks within the caves and makes sounds that mimics the original Cave Dweller, it's always watching you from a far no matter what cave you're in, it is just waiting to scare you and attack, so always watch your back and if the Nightmare is there, don't get too close and be sure to keep your eyes on it at all times or you might just lose your world...
This mod is a separate entity that makes sounds spawns in the caves. Don’t get too close because this entity is not friendly.