How to Get the Deltarune Egg and the Secret Behind the Joke Item
Here's my playlist for Deltarune related videos: https://www.youtube.com/watch?v=IpjPJsVRibA&list=PLpJ1vgAUKivJnTIYBIlAsUABw930cjwWR
I've gotten to the secret room twice now, so I'm pretty sure this method works. If it does or doesn't work for you, let me know and maybe we can figure out if there is more to it.
GimmickEiGhT's video showing how to use the secret key item and his theory for how it's related to Chrono Trigger is here: https://www.youtube.com/watch?v=eoaXbeow02A
I tried out some of Aigarr2000's suggestions from this reddit thread. While that reddit user reports being able to get back to the hidden room, I saved after saying "No" so I may not be able to.
https://www.reddit.com/r/Undertale/comments/9t61my/deltarune_spoilers_egg/
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#1 Marmaduke Fan currently has 3,024,057 views spread across 155 videos for Deltarune. His channel published over 3 days worth of content for the game, or 3.96% of the total watchable video for Deltarune on #1 Marmaduke Fan's YouTube channel.