immersion into my hyperfixations

let's start with some facts that we know mostly for sure (according to wiki):

1. "potentially any lightner can open a fountain, by PROJECTING THEIR WILL into a SHARP OBJECT and PIERCING the ground with it." —

["that is when i realized this power...

this power of the WILL

this power of DETERMINATION...

is this something that all lightners possess...?

if one was simply determined enough

could not anyone make a dark fountain?" - queen, prior to her final battle]

moreover, based on the quote above, “DETERMINATION” and “WILL” are used as synonyms. In other words, they are essentially the same thing in context of deltarune and furthermore i'll also refer to them as "INTENTION". (english is not my native language, but it does make sense to me that both determination and will are a strong intention to do something).

2. "apparently, kris is the ONLY one capable of sealing dark fountains, doing so using their soul." —

[furthermore, only kris can seal the dark fountain...

so if you don't accompany us, you won't make it home! - ralsei, chapter 1]

— supposedly it depends on determination aka strength of person's will aka intention.

3. "SEALING the chapter's primary dark fountain INCREASES the party's lv." which is exactly what happens after each fountain is sealed, yep.

4. when determination increases, all stats TEMPORARILY RISE (as our only scientific example with known stats before and during determination jump we have undying undyne from undertale: hp increases from 1500 to 23000, ATK INCREASES from 50/7 to 99/12, etc., but i'm pretty sure it's a universal rule for every character that can experience this).

5. as far as we know from the undertale, the amount of damage during an attack depends very much on the INTENTION of the opponent/player and increases greatly with WEAPONS (moreover, neither kris nor frisk have ever attacked without at least some kind of weapon, there is a possibility that this is due to the conditional rules of battle in the toby's worldbuilding) and lv. this pattern can ALSO be seen in deltarune.

6. it’s obvious, but still a fact: the battles in undertale work different compared to the battles in deltarune's DARK WORLD.

7. it's also obvious and little silly, but with the start of a fight the world: DARKENS, gets limited to the boundaries of the battle zone, and begins to dance according to its own bizarre rules.

(+8. bonus: when entering the dark world, we see several specific patterns, and IT SEEMS to me that the dark worlds created by kris and the dark knight have certain patterns of initial locations that differ from each other. if I'm not mistaken, then immediately after jumping into the fountain, you can determine who created it. in addition, it is kris' fountains that are most similar to the original grand fontaine, to which ralsei is attached.)

THEREFORE, considering all of the above, it seems to me that when a fountain is opened with a strong jump in intent, which leads to an increase in stats, and attack to the light world (in way of hitting the ground) with a weapon, a sort of "battle" is created. however, due to the fact that the fountain is created through intention aka determination, it takes a stronger determination to close it — this is where kris' soul of determination comes to the rescue, which is kinda of a joker in the deck with the strongest stats on this front. this would explain why almost anyone can open the fountains, but only kris can seal them (as far as we know, at least; because why try to fight back with a jack if you have a joker who is definitely more guaranteed to win the fight).

it would also obscure why the battles in undertale and deltarune are so different in the plot sense — because they are battles inside of a bigger abstract battle.

perhaps this is why ordinary dark fountain creations cannot enter the light world, because they are in some sense "attacks" that are devoid of magic outside of the "battle". they are tools like objects, but they gain their will due to the release of magic during a "fight" by the side of one who opened the fountain with their will/determination (like, say, ralsei's scarf is just a scarf until you join the battle and then he starts moving and attacking). then the dark worlds itselfs are technically not exactly battlefields, but rather its game windows of sorts.

it's always been kind of obvious from some angle, but this is the first time i've cared enough to formulate a theory properly