(Before we begin, note that I now believe this idea to be faulty and outdated. See the bottom for more details, and read at your own risk!)
First, to catch you up to speed: The Shadow Mantle is an item which is introduced by name in Chapter 2. When you enter Castle Town and talk with Seam after having defeated Spamton, he talks about the Shadow Crystals, a seemingly important object that will appear throughout each chapter. He mentions that, for the next Shadow Crystal, you will need the power of the "Shadow Mantle" to defeat a certain adversary, before looking around his shop, not finding it, wondering if it has been stolen. But, was it..? And by who?!
I posit that the answer is in plain sight! Look no further than this moment in Chapter 1.
Well... Perhaps not in plain sight in this exact frame, but this is the precise moment in which Ralsei, the fluffy boy of champions, changes his appearance. From his introduction in Chapter 1, where he starts as a mysterious cloaked figure, shortly before revealing himself as a dark, shadowy critter. And I use the word "critter" because he channels SO much critter energy. Let's look at a simple frame of Ralsei's shadow form in Chapter 1.
The skrunkly bunkly-- OK I must distract myself from the bit. Next, compare it to his "Real" form, which he reveals shortly after "mumbling to his hat" in the first image.
Notice how the dark form's facial features are "obfuscated" in a way. Obviously, it seems like the dark form is clad in a shadow of some sort, but the eyes, mouth, and other facial features are either simplified or not present. This entire moment with Ralsei pulling down the hat is weird. And perhaps you already realized where I'm going with this.
So... What is a "Shadow Mantle", anyhow? Well, the shadow part is obvious, at least. But a "Mantle", as Google puts it (Yes, that is how I conduct research), is an article of clothing similar to a cloak or shawl. Basically, something you wear over yourself. So... It's something you wear, and the name implies it's kind of shadowy!
Wait a minute
Yeah, so I think there is a nonzero chance that our alleged Shadow Mantle thief is none other than Fluffy Boy Ralsei himself!!! Which might be a shock. A strange character for sure, but now I'm trying to convince you that he is also a HARDENED CRIMINAL?!?! Well, phooey! The very idea of such a thing raises a LOT of questions, so I will lay them out now.
So, about that first thing. The connection I've been trying to make is pretty obvious: I believe that Ralsei has the Shadow Mantle, because I believe that it is what he has been using throughout the entirety of Chapter 1. The strange fact that Ralsei has both a "light" and "dark" form is an effect that I believe the Shadow Mantle to have. As the name suggests, it is an article of clothing and has something to do with shadow, so it would only make sense that, if worn (or "Equipped"), it would grant the user a new, shadowy appearance.
Second thing. Obviously, the implication is that if Ralsei stole the shadow mantle, he must have done it at some point before meeting Kris and Susie in Castle Town. Obviously, the specifics of such a thing can only really be speculated about, but Ralsei is already presented as a "free agent" in the story, not necessarily bound to Kris's party, or even to a particular Dark World (See: My mans just goddamn Appears in the Chapter 2 Dark World, from the same direction that Kris and Susie enter). Plus, while there would certainly be some time pressure if the unused classroom's Dark World was "new", the strange and hard-to-interpret mechanics of Dark Worlds leaves room for Ralsei to somehow have both the time and a means to snatch it. However, fitting this in with observations further below makes me think Ralsei probably was under some tight restrictions prior to taking it. (If you believe the Horns theory, you'll probably know how to fit it together with this.)
Third thing. Seam says it was stolen, but seemingly doesn't identify Ralsei as the thief?! This is even more bizarre if the assumption that Ralsei's "dark form" is the Shadow Mantle's doing, as that would mean that Seam surely would have noticed the change... Right? Well, while I do suggest this just to keep the theory alive in the face of this indicting evidence to the contrary, I think the way the shops in Deltarune have been presented thus far indicates that perhaps Seam can't actually see us, while "we" can see him. Firstly, note that (TO MY KNOWLEDGE OPEN TO CORRECTIONS) at no point in *any* of the shops in Ch1/2, are any of the main trio implied to be physically present in the shop. All we see are the Darkners, the menu UI, and a simple set of dialog choices which "we" choose to engage in conversation. Any information exchange between the Darkners is initiated by "us", the SOUL, and while it can be seen as standing in for Kris and the party, the fact that it is used as the primary means of interacting with the shopkeepers as opposed to the party interacting with them on the overworld implies that the shops are a different kind of "setting" from the overworld, one which operates on different rules, and one which may not even need Susie or Ralsei to be present for it. This can obviously be linked to the notion that Shops function differently in Undertale as well, but I don't think it's a necessary connection to posit this strange justification for such a seemingly blatant discrepancy. (Addendum: I remembered that there are overworld shops, such as the CD Bagel shop, which are presented differently from the Seam, Spamton, etc. shops!)
Fourth thing. If Ralsei has the Shadow Mantle, why doesn't it show up in his inventory? The explanation to this one also relies on some assumptions, but at the very least they are precedented assumptions. Firstly, note that one's belongings and one's "inventory" do not necessarily match 1-to-1. You can see everyone's "equipment" in the equipment menu, but it doesn't seem to meaningfully map onto how they visually look in the overworld. When you equip the Chapter 2 Mannequin, you do not visibly see a Mannequin appear. So, in the case that the Shadow Mantle is treated similarly as a "personal possession", it may not even have reason to appear in the menu. But, what about the Key Items menu? Well, this is something a bit more interesting: I'm inclined to believe that, if Ralsei has the Shadow Mantle, it might not appear in the Key Item inventory, as that could be simply Kris's Key Item inventory. I haven't verified it myself, but I'd like to point out the Manual that Ralsei gives you after the tutorial fight. From what I recall, even though Ralsei has the Manual, it doesn't appear in the Key Item inventory until Ralsei gives it to Kris specifically. If my memory is correct there, I think this point is properly addressed. If not, well, I guess I'll know eventually.
Now. It's time for us to enter...
What in blazes even is the Shadow Mantle?! Why did Ralsei steal it in the first place, why were they wearing it through most of Chapter 1, why did they take it off at the end of Chapter 1, and what in [[HIM]]'s name does it even do?! Well, let's try to use our inside voices. Obviously, we don't have much information to work with. Which is why, even if you agree with my point up until now, you can disregard this stuff and come up with your own interpretation. So, let's begin!
Well, let's start with a no-brainer. Why would Ralsei want to go so far as committing petty larceny to get such an item..? Well, obviously because it has an important use! If there's some serious stuff this thing is capable of (which I believe there is beyond just being "a cool equip"), I think it would make sense that Ralsei wants it. We already have lots of evidence indicating that Ralsei is operating on some sort of agenda beyond our full comprehension, and he's capable of doing things outside the scope of our observation. I think, at the very least, if the Shadow Mantle is all it's cracked up to be, it's rather believable that Ralsei would want it for his own machinations, whatever they may be.
I believe that Ralsei is wearing the Shadow Mantle for the wide majority of Chapter 1. It's kind of the whole basis of my theory, actually. But what is it doing for him? So far as we can tell, there isn't any obvious ability it grants Ralsei while he's there with us. But its effect is seemingly so important that we will be entirely incapable of defeating a future opponent holding a Shadow Crystal unless we have it on us. Is it a combat item? If Ralsei has it, perhaps not... But, even if Ralsei doesn't have it, either way I would like to toss some flat-out guesses of what it might be capable of.
First guess: The Shadow Mantle is an object capable of letting you perceive things in the Light World. I say this because of its connection with the Shadow Crystals, items that seemingly offer a "glimpse" into the Light World. It wouldn't be that much of a stretch to say that the Shadow Mantle enables a stronger version of that, and it would make sense that if this were the case, it would be very useful to Ralsei for keeping tabs on the status of the Light World, even if not completely.
Second guess: The Shadow Mantle enables teleportation between Dark Worlds. Yeah, that thing about Ralsei just kind of... Appearing in Chapter 2's Dark World? He doesn't follow you out of the Castle Town Dark World, but he also doesn't directly join you into the Lab Dark World either, instead somehow following you only after you've already engaged in the first battle against the Werewires. Plus, Ralsei is seemingly unaffected by the "exclusion" from Dark Worlds, not having any issues with "compatibility". Obviously this can be explained by something other than the Shadow Mantle, but at the very least, this is a tangible, actual explanation for how he could do it.
So, now I'm going to combine this information with my "game dev intuition", which is to say, what I personally think would be cool if it were the case. If the first guess is correct, I posit that it could be useful against the Chapter 3 Hidden Boss not by direct combat usefulness, but rather by allowing you to read the state of the Light World in the middle of the fight. If the Dark World is going to take place in the Dreemurr living room, then there is one thing present there which could be very important: The TV. That thing is on. And, my game dev rule of cool tells me that nothing would be more awesome than a Dark World boss involving looking at the TV set to figure out what picture it is displaying, and using that as crucial information for defeating the boss. Be sure to pass this on to your friends to will it into reality.
If the second one is true (And both of these guesses aren't mutually exclusive, it could be both!), then at the very least it would explain Ralsei's ability to traverse, survive, and sense the many other Dark Worlds. Again, this could be because of some yet-unknown attribute of Ralsei we have not been clued in on yet, but at least the Shadow Mantle theory provides an actual possibility we can rationalize about.
I felt it necessary to offer these possibilities before moving onto the final question: Why is Ralsei wearing it in Chapter 1, and why does he take it off at the very end in the way he does? Well, truth be told, this is honestly a hard one to answer. But, it's a hard one to answer even without the Shadow Mantle being involved. Ralsei is dark one moment, and then... Light the next? What be uppin' with that? Well, let's address the likely conclusion most people came to: Ralsei wanted to "hide" his identity at the start, and then "reveal" it to Kris and Susie once they were established friends. But... What necessarily is there to hide? The only way *that* explanation of the reveal makes sense, to me, is if Ralsei is Asriel. But I'm heavily inclined to believe that the two are separate until further notice, and that Ralsei is a Darkner like he purports to be. (Plus, being a Darkner would mean being subject to the full effect of the Roaring, which I think would explain part of his super visceral reaction to Berdly's attempt at making a new Dark Fountain in Chapter 2. I think it might be possible to gloss over that aspect if you're under the interpretation that he's a Lightner, or some other type of being.) But, if he's not Asriel, then why the moment?
Well, while it's not airtight, I do at least have an explanation to offer: What if Ralsei showing his "Dark form" at all was a mistake? Note that Ralsei was initially wearing a hood when he was first introduced. That was his dialogue sprite when talking with you, and that was how he depicted himself in the prophecy.
Shortly after explaining the prophecy, Susie asks him to take off the hood. He simply replies "U-Ummm, alright..." before taking it off. I believe he was expecting to keep the hood on so that he could use the Shadow Mantle without raising any eyebrows, but was genuinely taken off guard by Susie's request to lower it, realizing that in one way or another, it would be revealing information. So, he had to play it off as the way he looked for the majority of the chapter, and was looking for an opportunity to remove it. I believe he chose the end of Chapter 1 as a good time to do this, being a sentimental moment wherein he could pass off his taking off the Shadow Mantle as a show of friendship and vulnerability, rather than a strange change in appearance to be called into question. But hey, that's just a theory... A GA-
(1 Day Later...)
So, I tested the waters with this theory out in the open somewhere (it was just Andrew Cunningham's discord server), and I think it's pretty safe to say the verdict regarding this theory is... *drumroll*
Yeah, so you feed your theory to a space that trades in Deltarune theories and, as should be the case, you get your eyes opened. I'll keep this theory up for preservation, but I've been pretty thoroughly convinced this one's a dud. I'll list off the most important criticisms and questionable aspects now:
So that's it for the Shadow Mantle theory. I probably haven't covered everything of relevance to it, but I wanted to start with this theory first because it was basically my first "brain blast" moment while thinking about Deltarune's mysteries, primarily after watching Molly's first Device Theory video. I hope this was an interesting read! And see you for the next theory.
Sources used: