I've compiled both of my secret boss videos into one. Both secret/hidden/ultimate bosses are variants of Spamton Neo. I love the bits of lore that we can gleam from these. Where did Mettaton NEO's body come from?

Normal Run:
To unlock this boss you need to retrieve the empty disk from the basement of the Queen's Mansion. If you then give the disk to your good buddy Spamton, he will overwrite the contents of the disk and ask you to return it to the strange looking machine in the wall. When you bring it back, the fight will trigger.

Genocide Run:
The conditions for triggering a genocide run are very specific, but this Google Doc has detailed instructions on how to do it.
https://docs.google.com/document/d/1A...



Genocide Run Guide and Summary:    • (OUTDATED) Deltarune Chapter 2 - Geno...  
Series Playlist:    • Deltarune