In Deltarune, they live in the Lake next to Hometown. In Chapter 5, Normal NPC is found at the Festival selling "fried dough ball[s] made of Octopus". That Chapter's Weird Route involves the Lake in a way that Onionsan could intervene to prevent, so soon after the chapter's release, it became a theory/joke that Normal NPC killed Onionsan and turned him into takoyaki as to allow the Weird Route to progress, with the words Normal NPC repeats to the player in Chapter 4 implied to be Onionsan's last.
