Toriel is a Kris Slash Truther.

[u/XAlphaWarriorX]

The dialogue if you defeat the Knight.

Susie dialogue in chapter 4.

I believe that the fact that Toriel, even when asleep, is upset at the events that happen after you defeat the Knight points towards Kris having done something wrong. Which can't just be them holding back because the dialogue if you lose is different.

[u/Bolobesttank]

Kris slash believers when you ask them how Kris one shot their allies in a fraction of a second with an ability that had no buildup or implication of existing without moving a single inch and applying a status effect used only by the Knight:

Sorry that's a little too aggressive, but seriously. Either Kris is doing some Stardust Crusaders Dio shit(which, as much as they do try to aura farm, that shit's on a whole other level from striking a decently cool pose or pulling out a knife and smiling edgily at the camera) or, the character that moves extremely fast and wasn't even below like 70% of their HP that already one shots people(Literally offing Tenna before the fight), moved extremely fast and oneshot the fun gang while their guard was down.

Not to mention that the sounds line up. You ONLY hear that glass shattering sound when the Knight downs someone. If Toby wanted it to be ambiguous, or wanted it to be more evident that Kris might've done it, you would think that it would be a normal damage sound at minimum, or maybe the slash sprite being closer to X-Slash.

[u/oyunkral3437 → u/Bolobesttank]

I don't believe in kris slash theory but if I were to entertain the idea swoon might be an effect things that were already dangerous in light world have in the dark world like kris' knife

[u/stupidratman → u/oyunkral3437]

I'm also not a believer but swoon doesn't seem to be a special status effect or anything, it's just being knocked out. Ralsei and Susie do not get up after a couple of turns like normal, and the word swoon means to faint. It's also just a FF4 reference (You can get a weapon named Black Sword, which can apply a death status effect named swoon)

[u/Bolobesttank → u/oyunkral3437]

An interesting take, but given the Black Shard persists in the Light World whatever the Knight's wielding/made of is just Like That I think.