In a hole in the ground, there lived a nerd

Yes, I know, Jungian psychology is a horse beaten into a pulp, but it lingers still in creative writing and narrative tropes, so I will continue to bring it up when appropriate. It is at the very least relevant for this given the overall story structure of the movie. Just wait until I bring in metamodernism into the discussion plate--

I'm gonna skip on a lot of background to keep this short, but I'm sure you're familiar with the idea of an Ego, an Id and a Superego, right? Freud stuff. Well, Carl Jung expanded on the concept a bit and called them Archetypes.

There is the Self, the ego, the conscious mind, which is you in the intimacy of your own mind and self-perception. There is the Anima or Animus, which is your unconscious mind, the missing halves of yourself; for example if you're a man, then the Anima is your feminine side (yes it is an extremely binary system, this was made in the 1910s what did we expect?). They often appear to you in dreams and assist in individuation. There's the Persona, the masks we wear in day-to-day life to navigate society. Some are closer to your Self, but none of them are genuine, there is always a rift between the perception of the others of you and you as you truly are.

Aaand there's the Shadow. That is the part of your unconscious mind where all that is repressed, rejected and disavowed by your Self goes. It's the Other within yourself. Things that society tell you is inappropriate, wrong, things you internalize as negative and thus keep separate even in the privacy of your own mind.

According to Jung, it's in the Shadow that several complexes live, because the denial of the Shadow's existence and the projection of it onto others cause conflict. To cure certain psychological ills, you must face your Shadow, be aware of where it lies and attempt to either integrate or transmutate it.

To integrate the Shadow is to face all those things you deny about yourself and accept them as a part of yourself in a conscious manner. If your Shadow houses your anger because anger was not a safe emotion to have, then you must embrace it so you can learn to manage them in a healthy way rather than be haunted by it. To transmute the Shadow, you must deconstruct your own view of your Self and how you formed it, break it apart and incorporate that which forms your Shadow into it, so you may become a Whole person. This process is tied with alchemical visuals and terms for him, because there are a lot of overlap in the concepts.

So back to Iron Lung: Simon is haunted by this Shadow of himself, twisted and mutated almost beyond recognition. A creature is about to burst forth from his throat, a second mouth that isn't his. Choking him out, stealing both his voice and his mouth. As a reminder, the mouth is one of our first connections to the sensory, the physical world, our teeth are our first weapons and the way we eat, consume, sustain ourselves. There's a reason dreams about teeth falling off are extremely common and often tied with anxieties and feelings helplessness, lack of agency. It has no eyes, he's blind, another layer of helplessness.

Also, is that another skull budding out from his torn left arm?? The fangs are undeniably there, and all over his chest too.

We know already that the Shadowman is what Simon could have become if the blood had it's way with him. Actually for all we know, that is what his remains look like, regardless if the Blood Tree claimed him or not. But let's look at it from a Jungian perspective.

If the Shadow represents what is repressed and denied from the Self, then I can see two main paths we can take our analysis.

One: Simon's projected fears regarding himself, either past, present, or future. We can't discard time bullshit here after all!

Simon fears death, and it manifests as this rotten, macabre corpse. He's seen the skeleton by the time he's first seen the Shadowman, and it's not unfair he associates hollow eyesockets, odd tubes and sharp fangs with the bottom of the ocean;

It's also not unfair to assume he knows what a half-decayed human body looks like either, which only adds to the projection;

Simon also fears a stale existence, a fate worse than death. He's not content in accepting the world after the Quiet Rapture, whether it be accepting extinction or scraping for survival in the ruins of the universe. This creature is alive or dead or something in between, and he refuses to figure out;

Simon fears and abhorrs his own helplessness in his situation, a man muzzled and trapped by the powers that be. It manifests as a version of himself that is voiceless and blind, obedient in its helplessness;

Simon dislikes his own violent side, the 'butcher' in him, something he has rejected. It manifests as this creature of vicious teeth and drenched in blood. Rage is also often associated with blindness;

Two: The blood is projecting at him, for him, retroactively. As I said, we can't discard the time bullshit.

We know that the Pinhole God sees him as 'the butcher' and names him as such. While its assessment may not be complete, it's not entirely unwarranted. Simon did do all that murderin'!

The voice of TGWTYCW accuses him as such too. Whether it's the blood ghost of a dead man that was assimilated into the ocean, or Simon's own guilty conscience attakcing him, or a mix of both, it doesn't really matter. What matters is that Simon feels guilty, he bears that weight regardless of how much of it was his fault, his agency, his choice.

That's a part of him which the Pinhole God sees, recognizes, and will bring to the surface, Simon's opinion on the matter be damned. No matter how much you push it down, the Shadow will never leave you because it is you.

Which is why Simon's "Fine, you want the Butcher? Come on!" in the ending matters so damn much. That's not regression, that's progress! It's facing this shameful, repressed side of himself and using it again. Whether he's doing it selflessly or as a final "fuck you" just for his own gratification isn't the point. It's about a thing that he constantly pushes down, lashes out against, rejects, but now is embracing again one last time. Not because anyone told him to, not even because he needs to -it's already game over for him regardless if he fights back or not.