After getting all the stars the glitching and distortion of the entire Eternal Plexus you (Mario) get transported to the Engine Room, a completely dark room with a wooden floor and a glitchy block called The Core. In the middle of the room is “///.;” who is standing looking backwards

“[MARIO. IT SEEMS WE HAVE FINALLY MET. I HAVE BEEN WAITING SO LONG FOR YOU. AT LAST. AT LAST. MY SUFFERING SHALL END. AT LAST MY SALVATION IS HERE.]”

It spoke in a robotic accent but do to it being a personalized AI it sounded like Mario do to personalizing to him