The CORE is a region between Hotland and New Home, found after the MTT Resort. The CORE was built by the former royal scientist, W. D. Gaster, before he fell into his creation. The layout of the CORE can be rearranged at will which is why Alphys's map of the location is inaccurate during a neutral/pacifist run. Mettaton is fought in this location. It generates power used by the entire underground.
The CORE is populated by mercenaries (Madjick, Knight Knight, Final Froggit, Whimsalot and Astigmatism) paid by Mettaton to kill the human.
The only item obtained here is a Glamburger found in a trash can.