I've been working on an animation for the last 8 months. It's almost done. I just need to finish one last scene and fix a few things. Those pics are some of the shots I did.
A brief note: this theory is a reformulation of an analysis I did on the Roaring a few months ago. The original one includes some additional details I feel aren't as necessary for this theory. Feel free to read it here
There's a thing I see almost no one talking of, and it's about the Earth in the Prophecy and it's deep connection to Dess, possibly explaining how she might have transformed into the Knight.
Rereading different versions of the Legend the other day, I noticed a detail: the Heroes only appear AFTER a calamity occurs – The Roaring.
So why does the Fun Gang exists if nothing has happened yet? The thing is... it already happened and nobody realized yet... it's the Roaring Knight itself.
And no, they're not responsible for causing the calamity (as far as we know, it's relatively easy to create one, you just need enough DETERMINATION), but they would be a smaller-scale, walking version of the Roaring. After all, their resemblance to the Titans isn't a mere coincidence.
The Prophecy can give us an idea of ​​how Dess became this thing.
An addendum: I won't delve into how Dess initially fell into the depths/shelter/dark fountain or whatever happened to her to PHYSICALLY become the Knight, cuz it's not the focus of this theory. If I had to guess, I'd say she got trapped in some empty dark world, fell down, and her own determination kept her alive. Over the years, this determination accumulated, and got solidified with her ice magic, becoming the black shards that make up the Knight's armor – which is also what keeps her body intact and unable to completely melt from the accumulation of DT, yet still "malleable" enough to shapeshift.
SOULs are the culmination of a being. Without them, you are incapable of feeling anything. Then what if you kept the SOUL, but lose their light instead?
When the Earth is pierced, it causes darkness to leak out and it takes form of creatures known as Titans. The Lightners are left alone, LOST in an endless night.
Something that reinforces this better is OST 97 - "But the Earth Refused to Die", which plays during Undyne's monologue during No Mercy Route in Undertale, where she literally refuses to die and transforms into Undyne, the Undying.
You could go further by comparing excerpts from the lyrics of "Raise Up Your Bat" with the prophecy – considering the anagram EARTH - HEART:
When the demon heart is crying/And the blood is gushing bright
(When the darkened earth is roaring/And the fountain fills the sky) - Dess' SOUL losing its Light, opening space for Darkness
When your hope is slowly dying/And your future's lost its rights
(When your HP is slowly dying) - has fallen down
Come follow me into the dark
With your heart as the ark
Which shall shine you the way
Because I'm with you in the dark
With your heart as my mark
Which shall guide you the way, through the waves
(Darkness often represents negative, intrusive thoughts or depression, and sometimes... It's difficult to get out of this alone, needing an external force to help; a guiding light)
Now for the best part... how will Earth, or rather, Dess, be saved? It's not as simple as sealing a Titan.
December is commonly known as the "Lost Girl", except that being lost has different meanings for different situations:
Susie is lost because she doesn't feel like she fits in anywhere
Noelle is lost because she doesn't know what to do on her own
Ralsei is lost because he doesn't know who he is or what he should be
Darkners can flee if attacked; You'll never be able to have them as RECRUITs again - they're LOST
Dess is lost in the Depths/Code - she doesn't know where she is, she doesn't know what to do, she doesn't know who she is anymore - she's not capable of getting out of this alone... if it could, she would have already done it
Back to Undertale once more, during the final battle against Asriel Dreemurr, we encounter "Lost Souls", hopeless versions of our friends. In order to SAVE them, we need to remind them of who they were.
I believe the same should be done with Dess, helping her find herself, to remember who she is.
I don't think Dess being "trapped in the code/depths" is literal, but rather metaphorical, and in reality she is just buried within her own mind, surrounded by bad, negative thoughts and feelings... surrounded by darkness, as the Roaring Knight, without any light to properly guide her.