Plot

from what i got, this is his original him that was before he split into multiple people. he is probably locked deep inside his unconciousness considering we didnt see him interact up until this point, and only got back due to the trigger of bottom being kidnapped. which resulted in locking the bottom in other realm that the first mugo made up. its either in mugos unconciousness or he has ability to make new realms considering hes god afterall.
bottom is "safe" there but that place is probably not meant to him n he is using him as placeholder for the first bottom which he failed to keep safe. and the first bottom whos dead wants him to get over allat so he tried to use this current one to communicate it. went terribly wrong. honestly no idea how the first bottom is able to communicate but its most likely that first mugo is refusing his soul to go.