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-Shadow monsters made of fear.

I think there is a shell to break, but it's not that of an egg. But rather a cocoon. I believe that Ralsei is some kind of titan spawn. He has so many things that separate him from other darkners. Also Flowery calls him an impossibility. He's a titan with sentience and emotions, something that should be impossible. If you ask me some of the titan spawn look similar to bugs. My theory is that when Ralsei first came into existence, he was a mindless titan spawn. Consider this his larvae stage. He's basically a caterpillar. Possibly due to Gaster, the Grand Fountain, and/or the prophecy, he gained awareness. He then dawned the robe and overall form loosely based on Asriel. He entered his cocoon, and that's the form we know. However I think you have figured out where I am going with this. Ralsei will enter his final state, his imago stage. His mind will be erased and become nothing more than the mindless titan he once was. Only now he can do some real damage. Of course Susie and Kris will have to stop it. As foretold as the final tragedy. Except they won't kill the titan, they enter the titan and reach out to their friend, Ralsei. Eventually they react him and the outer shell crumbles leaving the three heroes tired but alive, well, and together. They finally defied the prophecy.