The Dweller called Nightmare.Jar is finally for 1.20.1! He behaves like a evolved Cave dweller from Cadentem. (In General he is a retextured Cave Dweller with different sounds and slightly different stats like more health, damage output and attackspeed). You can configure him to what you like. I advise you to use the mod Configured by Mr.Crayfish to do it in the main menu. You need to re-join your world when configuring settings while in a world as server.configs only apply changed settings when you renenter a world.
Credit goes to SwayleMC, Flamc04, GalaxyZGaming as they made the orginal.
Note: This is simply a port of the original for 1.20.1. I don't want or take any credit as the code is by Candentem and the Textures and sounds by the original creators listed in credits.