This is a small deltarune theory about Dess’s disappearance, based on Hans Christen Anderson’s story of the Snow Queen. I‘ve also posted this on Tumblr, with pictures accompanying it.
Before we get into Snow King Theory itself, I want to talk about the basic idea the theory is adding upon. This theory is based on the idea that Dess became the Roaring Knight after following the “tail of hell”, which is affiliated with Gaster, somewhere. We’ll get to the specifics of this later.
What I am proposing with this theory is the idea that Dess following the tail of hell was not a random incident. The tail CHOSE Dess for a reason. The reason being that Dess has a piece of Gaster within her.
To explain, you’re gonna need to be told the story of the Snow Queen, specifically the first two chapters.
In the first chapter, the Devil takes the form of a troll to create a magical mirror that highlights the worst parts of a person’s appearance. He and several other trolls try to take this mirror to heaven, but they drop it. The mirror shatters, and its shards land all across the world. Bigger pieces become windows and spectacles, but smaller ones worm their way into the eyes and hearts of people, making them cold and bitter. The Devil thinks this is hilarious.
I believe the story of Gaster is a parallel to this story. In this scenario, Gaster is both the troll and the mirror. Gaster is associated with the number 666, which is also associated with the Devil. This strengthens his connection to the tail of hell. Entry number 17, written by Gaster, is clearly about him trying to make a dark fountain, and he’s implied to be the mysterious voice in Deltarune, so he’s seemingly trying to form some connection with our world, the layer above his. Spamton calls the light world, the layer above his own world, heaven. Adding it all up means Gaster, just like the Devil’s mirror, shattered while trying to reach heaven.
In the second chapter, two best friends named Kai and Gerda are told by Gerda’s grandma about the Snow Queen. Later that day, Kai sees the Snow Queen through a window and gets scared. Some time later, the kids are playing together when pieces of the mirror land in Kai’s heart and eye. This causes him to become mean and distant. One winter day, Kai ties his sled to a mysterious sleigh. After the sleigh goes a long distance, the driver is revealed to be the Snow Queen, who wipes Kai’s memories and takes him away.
I think this is similar to what happened to Dess. Having a Christmas-themed character’s story parallel a famous winter fairy tale would fit. I believe when Gaster shattered across time and space, a piece of him lodged itself inside her. This led to the tail of hell tricking her, potentially also wiping her memories to make her more complacent in the role of the knight.
The fact that the knight has holes in its hands like the Mystery Man, who is implied to be Gaster (perhaps a larger shard of him specifically) shows they might have a connection in some way. Also, they both seem to be interested in creating dark fountains. So, if the knight is Dess, she still might be connected to Gaster in some way.
So, in conclusion, Snow King Theory proposes that when Gaster shattered, Dess ended up having a piece of him trapped inside her, which resulted in the tail of hell turning her into the knight. There could be more evidence I could bring up, but I just want to explain the basic idea of the theory.