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So, let's start by looking at the root of this whole theory: The song coming from the lake. I'm guessing anyone reading this post is probably already enough of a Deltarune fan to know what I'm talking about, but as a refresher, there's a lake to the right side of Hometown, and if you stand on a specific spot by its shore and wait for a little while in Chapters 1 & 2, you can speak to Onion, a returning character who first appeared briefly in Undertale. In Chapter 2 specifically, Onion will tell you about how sometimes, at night, they hear a mysterious song coming from somewhere deep below the water. They also say they think they've heard the song before, but are struggling to remember specifics, and they tell you they'll go to investigate it and meet with you again tomorrow.

However, when tomorrow comes, that never winds up happening. No matter how long you wait by the lake in Chapter 4, Onion will never come to meet you. Susie suggests they're just "busy", but considering how they specifically told you to meet with them again that day and the last you heard from them they said they were going to go investigate the song, it's hard not to suspect that something bad must have happened to them. As of right now, we still don't know. Making things even eerier is the mysterious striped bird NPC from the library (who's a whole other can of worms I'm not going to dwell on too much in this post) appearing by the lake if you leave the screen and return after trying to wait for Onion. They'll stand in exactly the spot you normally need to stand on to wait for Onion, and tell you this:

This is obviously pretty cryptic, but the most common interpretation of what the hell this weirdo is trying to say is that they're warning you not to WAIT by the lake again. "Don't wait." It would certainly explain why they're standing in exactly the right spot to keep you from trying. It's even possible for them to appear in this spot while it's raining later in the chapter, so being there to tell you "Don't wait" is evidently important enough to them that they're willing to stand out in the rain for it. Regardless, they'll be gone if you exit the screen and return again, and if you proceed to not heed their warning and return to wait by the lake again at the end of the chapter after defeating the Titan nothing obviously bad will happen. . . but now, Susie will hear the song.

So when Onion went to investigate the song, they vanished without a trace despite claiming they'd meet with us again tomorrow, and then when the striped bird NPC seemingly tries to warn us not to wait by the lake, failing to heed the warning leads to Susie being the next to hear the song. All signs seem to be pointing towards that song being something dangerous, and if Onion's disappearance is any indication, attempting to follow the song to its source is likely a bad idea. You know what this all reminds me of? Well, you already saw what this theory is called.

For those who don't know and don't have the time and/or patience to read that whole Wikipedia article, a siren is a monster from Greek mythology, residing out at sea and known for luring sailors to their doom with a beautiful, alluring song. Granted, the song in Deltarune is coming from a lake, which isn't quite the same thing as the sea, but do you remember how I mentioned that Onion claims they've heard it before? Well, guess WHERE they think they've heard it:

But don't you worry if this reasoning seems flimsy. I'm just getting started.

As mentioned earlier, Onion first appeared in Undertale, there going by the name of "Onionsan". You meet them in Waterfall, where an extended conversation with them occurs as you walk through a long, otherwise empty screen. Aside from a cameo in the True Pacifist credits, this is the only place they can be found in the entire game. However, what I'd like to call your attention to here isn't anything that happens in this conversation with Onionsan - it's what happens immediately after. You continue to the right, and on the very next screen, there is a scripted encounter (oddly disguised as a random encounter, but in actuality the game is programmed so this encounter ALWAYS happens on this exact screen and nowhere else) with a siren-based monster called Shyren.

The placement here seems awfully suspicious, if you ask me. Again, it's the screen directly to the right of the one and only place in the game you can talk to Onionsan. Meanwhile, in Deltarune, the one and only place in the game where you can talk to Onionsan is by the lake. The lake, which is located to the right of Hometown. And since the lake visibly extends offscreen and the song is described as sounding distant (either deep under the water, or on the other side of the lake) by both Onion and Susie, we can assume the source of the song is somewhere further off to the right, beyond the border of the screen. There's a clear correlation here. In Undertale, there's a siren-based monster to the right of where you meet Onionsan. In Deltarune, there's a mysterious song probably coming from somewhere to the right of where you meet Onion. This could just be an eerie coincidence, true, but it could also be a clue. And I'm not done yet.

It's time to take a bit of a sharp turn with this discussion, because I can't really think of a natural segue into this next point. It's time to talk about Raise Up Your Bat.

What does Raise Up Your Bat have to do with any of this, you might ask? Well, let's start with when it plays. As you know, it's the song Tenna makes you perform for the Lightners Live segment at the end of Board 2 in Chapter 3. Not much to note there, up until you remember the context this has within the narrative that Tenna provides to accompany the board.

Lightners Live - at least, within the context of Tenna's little made-up story - is happening in a place called "Atlantis Stadium". Within the board itself, you need to cross a sizeable body of water in order to get there, and I doubt I need to point out the obvious implications carried by the use of the name "Atlantis". Curiously though - and I'm actually just now realizing this as I'm typing it - Atlantis Stadium is never actually said to be underwater like what you'd expect from the name. Tenna even describes it as "floating", which is quite the opposite. This is a bit of a reach, but do you think maybe that could have something to do with how Onion hears the song coming from deep underwater, but Susie hears it coming from across the lake? I don't know, just some food for thought - again, this literally JUST occurred to me. Anyway, the point is, we're going to this Atlantis Stadium, a location out in the middle of a large body of water, to perform a song. And doesn't the image up Tenna uses to portray this stadium look disconcertingly ominous? That doesn't make sense within the context of the board, but it suddenly feels a whole lot more fitting if you assume it to be in some way representative of the source of the song from the lake.

Now, let's take a look at the lyrics of that song we perform in this distinctly aquatic location, why don't we? More specifically, the chorus:

Huh... is it just me, or does that feel subtly evocative of a siren song? The singer is beckoning someone to "come follow [them] into the dark," There's two different references to nautical imagery ("ark" and "waves"), and whoever the song is addressed to is being told to, essentially, let their heart guide them. Much discussion has been had about the obvious ways that last point correlates to the soul, and I have no doubt that such a reading IS intended, but looking at it through the lens of Siren Song Theory, it can also take on a more ominous double-meaning. Sirens, after all, are typically portrayed as taking advantage of the emotions and desires of sailors to lure them in. A siren wants you to let your emotions guide you - that is to say, they want you to follow your heart - so you'll be lured right into its trap. And one thing that's always bothered me about these lyrics is how they mostly talk about "your heart" guiding "you", with the "you" here being whoever the song is addressed to, but in only one line, "your heart" is instead described as "my mark" - that is, the singer's mark. In the context of the Siren Song Theory reading, this can suddenly make a bit more sense. After all, if the singer is a siren, and the "you" being addressed is its latest victim, there's a less common definition of the word "mark" we can use here that makes an almost uncanny amount of sense:

Now, as I'm sure most of you know, Raise Up Your Bat is also a song that is heavily implied to be in some way connected to the character of December "Dess" Holiday, Noelle's missing sister. Not only does Tenna mention her by name when he's introducing Lightners Live, and not only does the musical nature and punk aesthetic of Lightners Live generally seem to align with what little we know of her personality and interests based on her room, but the main leitmotif throughout the song is Lost Girl, a song heavily associated with Dess. What does this have to do with anything? Well, as I said, Dess is Noelle's missing sister. As it happens, there is also one character in Undertale with a missing sister.

That character is Shyren.

Shyren's sister, who we know from her grave found in Deltarune to be named Shyra, is one of various monsters who was sent to Alphys on the verge of death, being used as a test subject in her experiments with determination. The result was that she became one part of the amalgamate known as Lemon Bread (as it happens, also the one amalgamate to consciously lure you into a trap, disguising itself as a save point - another allusion to a siren's modus operandi, perhaps?)

It's not something most people would think about often, but since Alphys kept the amalgamates locked away in the True Lab for so long and never told anyone what happened to the monsters she experimented on, this would mean that, for a long time, Shyren was completely in the dark about what happened to her sister. Furthering the parallels to Noelle and Dess, we know Shyra to have been musically inclined from her gravestone in Deltarune calling her a "brave singer", just like Dess. Meanwhile, Noelle and Shyren both are said to be talented singers by other characters, but seem less confident in themselves about it.

Of course, in the True Pacifist ending, Shyren and Shyra are reunited at last, but Shyra has still been forever transformed into a bizarre, unnerving monstrosity - still Shyra deep down, but her mind and body have both been irreversibly altered. If the parallels are intentional, this may well be a hint towards Dess's true fate. A harrowing thought in some respects, yet in other ways oddly hopeful.

Now, with all that said, there is one last thing I would like to point out. See, aside from Lost Girl, there's only one other leitmotif in Raise Up Your Bat, able to be heard in the background during Kris's guitar solo before the first chorus. It's a rather strange leitmotif to include: None other than that of Mike. Thus far, only three songs in the entire game (not counting the cut stealth song) feature this leitmotif, the other two being Volume Adjustment and Catswing, which are much more obviously Mike-related. So what could this leitmotif be doing in Raise Up Your Bat?

Well, in all likelihood, Mike is some sort of microphone-based darkner, given the prominence of microphone imagery in his room, the fact that the fight against the three impersonator Mikes uses the microphone for a gimmick (at least in the PC version), the fact that the only trait all three of said impersonators' Mike costumes have in common is microphone theming, and, y'know, the name "Mike." Now, consider this: Where ELSE do we see microphones in Deltarune? Well, Tenna has one, which makes sense both because he's a TV host and because he's one of the characters most closely related to Mike. Ralsei sings using one in Lightners Live, which is another expected place for a microphone to be used, and we just established the minigame, or at least Raise Up Your Bat has implicit connections to Mike through his leitmotif. There's a microphone attached to the podium at church, but that's pretty standard for modern churches, it's not remarked upon at all, and it isn't even the same type of microphone as what all of the microphone imagery surrounding Mike consistently uses. Much more interestingly, though, are the two other microphones I could find in the game.

One is found in Dess's bedroom.

As for the other? It's perhaps the most clearly displayed microphone ever seen in the light world, and you can see it as early as Chapter 1.