I think the Shadow Mantle Holder possessed King somehow and maked him more agressive
I don't know how to put images here, but look at King’s cape and at the Shadow Mantle Holder…
Then say to me if it is or not similar.
If they're the same cape, how did it end up in TV World?
It... Are you forgetful? Have you not beaten King in a while? His cape... Flies off at the end of the fight. Assuming it is the shadow mantle, it's likely that it had some sort of property that let it get back home. Even Seam was confused as to how it got there, so if it goes unexplained, then we'll likely have no idea how it got home. Of course, that bit could be used as a counterargument, but I'll leave that up to you to make.
what is a cat meant to mean
Okay, here's a crackpot theory: After the fountain was closed but before the lights turned on, Kris put it (let's assume that it's a piece of cloth in the Light World) in their back pocket where they store their knife. Later, after they put us in the cage, they shoved that cloth as deep into the couch as possible so we never find it. That'd explain why Seam doesn't know about it, would be consistent with Kris's motivations, and wouldn't involve inanimate objects traveling on their own in the Light World.
cat
Toby says that King was influenced by the fountain.
The shadow mantle holder looks like a black mantle because they’re wearing the shadow mantle.
I think it’s plausible going off of Toby’s comments that King wore the shadow mantle, but not that King was possessed, especially since King still tries to kill the lightners without his mantle, and even threatens to kill his own people.
Seam owned it to begin with. How do you think it ended up in TV World? It’s not like that’s a question that this theory creates.
I think the mantle was stolen and given to King, perhaps by Jevil, to make him a more formidable enemy to the lightners, since he’s very weak, considering he’s the boss of the first chapter and strains himself so much that Ralsei needs to heal him even if you don’t attack him at all. When the mantle flies away at the end of the fight, it then gets retrieved by the shadow mantle holder, who can move freely throughout the darkness similarly to Ralsei, and they then guard it in the hidden game in TV World.
Can't find any holes in this explanation. I think it could work as well.
That being said, we also never get ralsei having free access across the darkness explained, so that needs to be explained first.
In regards to how I think it ended up in TV World: During my first playthrough, I didn't draw the connection between King's cape and the Shadow Mantle, so I assumed that the Mantle has been in Kris's house the entire time. In other words, Seam forgor 
. Now that I reread the dialogue, however:
I can't find it. Did someone... take it...?
Yeah, them being the same thing definitely makes sense.
So, to get into much less grounded speculation,
I think Ralsei and Eram (I’m going to say this name for simplicity) are made entirely out of darkness, not being tied to any object. As a result of this, they are very in tune with the minds of lightners, and do not have a static physical form. This is why Ralsei seems so visually different after Chapter 1, why he disappears into empty clothes when KO’d (which no other darkner does), and why he has connections to Asriel. Ralsei takes shape as something that would be appealing and familiar to Kris, for their comfort. Eram, on the other hand, tries to reinforce Kris’s dark, feared thoughts and emotions, as a sort of anti-Ralsei. This could also be interpreted as the reason the prophecy always shows Ralsei cloaked, because his appearance depends on who else fulfills the prophecy.
As for how these creatures exist, I think that they could be purified titanspawn, darkness tempered by light and thereby given the gift of will, but really that’s just a guess, and I don’t know. I just think the purifying thing should really mean something, because why would Toby put a choice in the game of whether or not to kill titanspawn if it was meaningless, why would he keep track of it, “When did you start being yourself”, etc
As for who would do the purifying, probably Gaster or something, of course it would be Gaster
Well, that checks out. Explains why they can move freely, it's like Splatoon ink.
Well, we've never really seen another darkner get KO'd, they either run away or straight up die in the case of titan spawn. Presumably, we never actually KO them, they just forfeit and run away when they know they're on the brink of being KO'd, or, in the case of K. round and Sweet Captain Cakes, heal, and in the case of C. round, get banished to the shadow realm of no return*
*Unless you have plot significance.
This is insanely plausible, surprisingly. The cliffs have the same """Eyes""" as the Titans in the prophecy, and the tower that spawns the titan in Ch. 4. There are theories that the roaring has happened before, and the cliff eyes are the biggest point.
Piles of Asriel dust aaaaaaaa
It's like everything on the internet leading back to 4chan, and every Minecraft YouTuber leading back to EthosLab.
(Joke comment)
I think Ralsei is a Goat Simulator CD that was on the closet
^^^literally📠
I came here to the bottom of the page without looking very well to the other comments… if anyone has commented on that, just read this. If anyone will comment about this, stay warned~
I think anything born on the Central World (Castle Town) will be able to go to any Dark World (which is proofed by Ralsei) and maybe, the Shadow Mantle was forged here, as it seems Ralsei’s kingdom was very powerfull or big. Also, if Ralsei is still the prince, where is the king?
Gosh, just found a comment saying something about the Eyes in the cliff…
My theory is that it was a forbidden place of cult to something, like an entity of Dark, etc. And it really seems that it has something that seems a bit religious, like “Only eyes blind by Darkness can see the way.”
It also seems very different from what is suggested to be the “Normal” (dominante) religion, like the Dark Sanctuaries, built in the place where the Church would be.
Imagine commenting without reading other comments. Couldn't be me.
That’s a fair point. I think being KO’d can’t mean much other than becoming unconscious or becoming too weak to fight, and I think we’ve seen darkners do that, so I believe the clothes thing is a special thing about Ralsei, but it might not be.
Ah, thanks for the evidence. Also, now that we know that titans can be summoned without it fully becoming the roaring, it makes it even more plausible that titans were in that area in the past. Not that the past roaring theory is impossible, but the past onset and reversal of a worldwide calamity without it being mentioned in the game would require a lot of explanation.