This section of IGN’s Little Nightmares 2 wiki guide will detail the game’s secret ending. You’re only able to unlock this ending by finding all of the glitching remains.
SPOILER ALERT: This page contains very important story spoilers.
After destroying the music box, Six will return to her normal size. However, whatever’s unleashed in the Signal Tower is coming after you now. So follow her and run. As the Signal Tower begins to crumble, you’ll have to jump and catch her hand.
Unfortunately, Six will betray you and let go of your hand, sending you tumbling down into the fleshy substance that was coming after you.
Once you’re back on your feet, head right until you see a chair. Then, climb on top of it. Mono will sit in the chair and, as time passes, you’ll learn that he was actually the Thin Man all along.
After this cutscene plays out, another one will start showing that Six escaped the crumbling Signal Tower through a TV. Once she’s back on her feet, a glitching version of her will appear in front of her. Then, the both of them will look down at a picture of the Maw, the location she explores in the first game, on the floor.