During Alvin's bangin' sermon, Susie mentions that Kris and Catti often performed occult rituals to try to summon a demon.

Now, in Chapter 5's Weird Route, Noelle says that she wants to go with Kris to a place no one has ever been to before (the lake's other side); moreover, she, Dess, Asriel, and Kris always wondered what was over there.

Perhaps Dess and Asriel did this too, but Asriel noped out eventually, leaving some of his fluff in there, while Dess went on. The primordial Dark World may have already existed by then. From the fluff and darkness, Asriel was born.

But could Dess have disappeared in there? I doubt it; the Shelter appears more likely.

In the UNUSED texts, which are heavily implied to be from Dess, this may be shown by the one from Chapter 3. It mentions a doorway and walking.

What is this place, anyway...?\
How did I get here?\
... as if I haven't asked that question a billion times already.\
I...\
I always did have that nightmare.\
Walking into the darkness,\
With the light shining from the doorway...\
Then the door slams behind me.\
And everything goes black.\
... is this that nightmare?\
... or was everything else a dream?

Another interesting UNUSED text is Chapter 5's:

I do have one thing, though.\
And that's memories.\
Memories of my life...\
Maybe that's all I am at this point.\
A collection of memory...\
...\
Then all I can do is... not forget, right?\
"Don't forget."\
I just have to keep telling myself that..

The second-last sentence is especially interesting, considering that the one calling Kris at the end of chapter 3, says: "Don't forget, Kris... you promised". If it is Dess, then how does she communicate with Kris?

Anyway. In Chapter 4, Alphys mentions that she has only seen Carol and Undyne go there before. She also mentions that it is a place for people to survive an emergency, but that she wonders whether a disaster happened there. The locks contain three entry fields; a pine tree, a police badge, and a DELTA RUNE. This in my view, implies that Carol, formerly Asgore/now Undyne, and the Angel have access to it. We don't know whether these specific entry fields existed prior to the 'disaster'.

Undyne is surprised by her own abduction. We know that Kris has a bond to the Knight. Carol is supportive of Kris, and Asgore is supported by Carol in his continuing investigation.

The Knight must be a Lightner, since they can open fountains; though the Knight may wear armour (as pointed out in Chapter 4, where the player can attempt (and fail) to make Kris imagine the Knight "take off their helmet"). The Black Shard stays the same in both the Light and Dark World, which is interesting.

I think it's the policed, lined-off area to the west. That is near the graveyard's forest, and Noelle mentions in Chapter 2 that she, Kris, Asriel, and Dess used to go there often. Noelle said that there was an incident where they hung out there and she was terrified, but got comforted by Dess.

Another detail I now recall, is that Asgore hasn't seen Ralsei before, and only by Chapter 5 remarks surprisedly he looks similar to Asriel, but is different (and Asriel has vanilla horns, not strawberry-coloured, as Susie says when you point out the family photo on the Dreemurr fridge). This is seemingly at odds with Ralsei having been there for "millennia", though time seems to flow faster in the Dark Worlds.

In the Sword Route, some areas are representative. The Island and Ice Palace Boards are unique, but the manhole dungeon exists in Chapter 2 too. If that means other areas in the route may also exist, then the area within the Shelter may indicate that it is connected to the Lake

1. Catti and Kris hold occult rituals where they attempt to summon demons.
2. On a fateful day or night, Asgore leaves the Shelter unlocked, which may have held a different purpose back then.
3. Kris enters the Shelter, using Asgore's badge for access.
4. Kris does one of these occult rituals while carrying Asriel's fluff, thinking of their hopes and dreams, and creates the primordial Dark World, using up their original SOUL and creating Ralsei from the fluff, and Kris' own hopes and kindness, soulless yet influenced by one.
5. Kris leaves without closing the portal; because they don't have a SOUL anymore, they cannot close it.
6. Dess explores the graveyard forest to investigate what Noelle was scared of, and walks into the Shelter, exploring it, and disappears there. She gradually gets consumed by the darkness, but promises herself to stay alive. Kris' original SOUL ties itself to Dess' body, and gains some of her personality. The SOUL banishes her spirit outside of the game's boundaries throughout a tunnel that leads to beneath the lake, much like the Sword Route's ending. Combined with Dess' body, the SOUL becomes the Knight. A monster with a human SOUL, born from pure darkness. This is all unknown to everyone.
7. Kris finds out about this, and want to retrieve their own SOUL, but also save Dess. They don't want evidence to be found of them having caused her disappearance. They don't want their own SOUL to disappear, and so Kris and the Knight make a deal. This deal is: bring the Knight a new soul and reunite it with another vessel so the Knight can bring forth the Roaring and live anywhere; and they will return Dess and Kris' soul.
8. Kris informs Asgore and Carol that Dess disappeared. Both are shocked, and all three go on to work together to figure out where Dess is. After Asgore and Carol enter the Shelter, they find there is a Dark world, and find the Knight, but fail to find Dess.
9. Eventually, it is clear that Asgore fails to find Dess, and he is fired from the police force. Rumours spread of the Shelter being an evil place, because of the dessappearance there.
10. Carol believes Asgore may yet be onto something, and so continues to support him. Traumatised, she keeps the room 'frozen in time', in the hopes that it will preserve whatever evidence is there, until they figure out a way to retrieve Dess' spirit.
11. Years later, Kris finally figures out how to summon a new SOUL in a ritual. They request to become able to open and close portals themselves, through the new SOUL. Now, they can search in new places in the Dark, and so make Dark Fountains.
12. The Voice, who is at a layer above the game but below the player's, hears Kris' summons, and creates the player's SOUL, Patience, but discards its vessel (possibly into the Shelter). This is where the chapters themselves start.