Here's Shadow Jevil, based on Undyne the Undying, cause why not, and you can even see him wearing the shadow mantle, based on eram. For ch1 of course, you can fight him at the end of the chapter if you only open his jail but don't enter, see ya and bye>:)

Torawareta rivu~aiasan, the Leviathan within the depths of the Third Sanctuary. Imprisoned by an anchoring chain and put under spell to destroy everything in sight by the night, thanks to the Knight and their Shadow Crystal, a new type of enemy it seems..

Culti the Ritual Cat, a witch secret boss you can find in the second sanctuary, and in the lightworld they are Catti's occultism kit, and yes they will use the lightners in a ritual of some kind, but you gotta wait to see it so see ya and have a good time

Time to meet Miss Florel, the Keeper of the Destroyed Garden. Yep you get want some of you want, a flower boss for ch3, and is basically a combination of flowey and muffet in terms of battle, personality, and design, hope you enjoy it bye >:)!!!!!!!!!!!!!

Meet Spamton EX, and secret form you unlock in somewhat similar standards to how you trigger Mettaton's EX form in Undertale, and funny enough, you get different items if you beat him, the Star Shades and the Exclusive Ragger, welp see ya and bye >:)!!!!!

Seam, the High Mage, a castle town ch2 secret boss, testing you to see if you are worthy of even wielding the shadow mantle, in which of course after the fight he can't find, but he does have something, a shadow crystal in his empty eye, welp see ya >:)!!

Meet Sheriff Buddy, yes you also get a Friend Inside Me boss, but instead of Friend Inside Me, it's Friend Corrupted Program, and Sheriff Buddy is actually a sweet guy when not a corrupted TV program, welp see ya and bye, enjoy these secret bosses >:)!!!!

Here is Miss Taki, a Mizzle from the Third Sanctuary, who asks for the party's help to free the Torawareta rivu~aiasan from it's imprisonment and control, done by the Knight themselves, can you free it? Hope you like the project so far and see ya >:)!!!!!

Here is Moonsie, a secret boss for a small portion in the library. He is based on creepy bedtime stories and uses the blue soul on you, and fun enough Noelle fights with you in this battle. I feel like this would be fun and i hope you find it that way too

Time to meet Eramb, you unlock under interesting circumstances, bye giving him the shadow mantle to free him from his stone form, but then he gets plugged ina nd is corrupted by Eram/The Mantle, and you and the party are sent into the game, welp see ya>:)