My concept has gotten a bit of attention so I'm making a second post for the other ideas for it.

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Character is a sensei, this chapter takes place in asgores flower shop. This character is based off of the little love note on top of asgores tv. This character is a essentially a sheet of paper, but it can fold itself like origami.

Other characters in the flower shop worship it on top of the "radio tower". On top of radio tower is a dojo of cultists for this secret boss. Once you approach it, it speaks in some kind of chicken scratch and folds itself into a ninja-like form, your soul turns pink and you are separated from susie and ralsei (original idea is that jockington and kris go to the dojo, but it didnt really make sense.)

This is the first time in deltarune where you are in 2 battles at once. Kris is battling the Sensei in one room, and after kris's turn, the camera moves to susie and ralsei battling the bosses cultists in the other room, where you take control of susie and ralsei for one turn.

The goal of the battle for kris is to survive as long as possible without being able to use items while susie and ralsei battle the cultists. (In this battle, like stated earlier, it makes you play kris's turn, then it passes you off to susie and ralsei and they have to defeat the cultists, once they do, they are reunited with kris and can properly defeat the sensei.)

Once you beat the sensei, its text goes from chicken scratch to a slightly more legible text and it gives a short speech about wants and needs before turning into a item to help you.

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Item for pacifist: "Dark Cloak + Shadow Crystal"

"Dark Cloak

A worn purple rag that confuses your enemies, given to you by the sensei of radio tower.

+7DF, +6 Magic, on first player turn, all acts and attacks deal double mercy and/or damage value"

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Item for genocide: "Ninja Star + Shadow Crystal"

"Ninja Star

A origami Ninja Star, the center of it glitters with power

+16AT, on first player turn, attacks put random status effect on enemy"

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Themes:

Battle theme for the boss;

https://soundcloud.com/haloboi3-yessirry/deltarune-chapter-4-secret-boss-concept

Area theme for dojo;

https://soundcloud.com/haloboi3-yessirry/deltarune-chapter-4-secret-boss-concept-2

Area theme for radio tower;

https://soundcloud.com/haloboi3-yessirry/deltarune-chapter-4-ost-030-radio-tower

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Concepts and designs:

Original Ai generated image, main design concept

Fanmade concept (in game) by Mayhemical

Quick sketch I made