DELTARUNE Chapter 3 has a secret boss, Roaring Knight, whom you can hope to defeat only if you have an S Rank. But it’s nearly impossible to do so without items and equipment from not only Chapter 3 but from Chapters 1 and 2 as well.
Nevertheless, Roaring Knight himself is not a secret, as he appears every time towards the end of Chapter 3 after you defeat the main boss, Mr. Tenna. Furthermore, the story continues to progress, regardless of the outcome of the fight against the Roaring Knight. What truly makes him a secret boss is the fact that you can find exclusive rewards if you do manage to defeat him.
