My version of Arthur Lester, or I guess now he's officially going to be known as dumpster Arthur (sorry man), has essentially the same storyline up until the Dreamlands, where the slight canon divergences start, culminating in the deal made with Kayne in Addison, with a very different "catch" as payment for John's return.
This is a bit of a long one, so click the readmore!
CW: cannibalism, autocannibalism, self harm, starvation, all canon typical shit
Starting with the prison pits in the Dreamlands!
Instead of giving in to killing and eating Faust after learning about what he'd done, Arthur held out a little longer by eating bits of himself. He has bite scars on his right arm and his legs, all avoiding the parts of himself that John can feel.
John was not exactly psyched about Arthur having to do this, especially because it takes a lot out of his friend; not just physically. He even tried offering Arthur his arm instead so the poor guy wouldn't feel it, but Arthur adamantly refused, because he'd already bitten John once before (their pinky) and was pretty horrified at how right it felt to have flesh between his teeth.
He doesn't talk to John about that, though, especially while they're still fighting about John's outburst about Faroe. John remains relatively in the dark about Arthur's internal struggle eating human flesh, even his own, since it's kind of hard to tell if he's feeling more fucked up than usual given their situation.
The altercation with Faust ends up pretty much the same way, Arthur broke and attacked him, forcing John to relive his death while he ate. John does start to notice something is off with Arthur psychologically at this point, though he assumes it's because Arthur has just killed and eaten someone for the first time.
They also do not talk about this because Arthur refuses (mostly out of shame and fear that John will think he's a monster haha)
Given how long Arthur was able to hold out before killing Faust, they're in the prison pits for about a month longer than in canon, and upon escape, Arthur doesn't feel the same amount of hope at their future.
Things proceed pretty close to canon at this point, they meet Kayne, they get their shit wrecked by the King, Arthur cuts his own throat, John gives himself up to save Arthur's life, etc etc.
The one big difference here is something the King says to taunt Arthur; he mentions that Arthur has come closer to knowing his form of love than any human ever has, and it has scarred Arthur permanently, even if he doesn't see it yet.
Now, to Addison!
Upon landing in the cabin and calling Kayne, Arthur is presented with a different option; get John back, safe and sound, memories intact. However, when asking about the catch, Kayne says something along the lines of "you'll have your golden boy back, but part of you will forever remain in the pit".
Kayne here just kind of wants to fuck with Arthur and watch him like a TV show, so this is more entertainment than anything power-seeking; Nyarlathotep (who I assume Kayne actually is) delights in cruelty and causing madness. Note, Kayne doesn't actually do anything to Arthur here, he's just kind of exacerbating Arthur's self doubt about his own humanity and sanity. No worse enemy than one's own mind, after all.
Kayne also does inform Arthur that John is in the Dark World! This obviously sways Arthur's decision quite a bit, and he immediately agrees to Kayne's terms, even assuming part of himself would be sent back to the prison pits, John not being alone in the Dark World is worth it to him.
Some of the main consequences to this decision!!
Arthur isn't really able to come to terms with John intrinsically being a part of the King in Yellow, and neither is John!
Arthur still very much views John as something that never was and can never be the King in Yellow, so he holds him to those standards, making some of their fights while in Addison have a different flavor to them.
In the same vein, John isn't able to have that "aha" moment of accepting his past as part of him, instead continuing to fight tooth and nail against the things he thinks are something the King would do.
As such, when Arthur snaps again and attacks Larson, and starts his descent towards bedrock in the mines under Addison, John is much harsher towards him, starting to see Arthur as the monster the King in Yellow is.
This culminates in Arthur killing Uncle, where another big problem makes itself apparent; the smell of blood makes Arthur painfully hungry, and he realizes this is what Kayne meant when talking about the part of him left in the pits.
Arthur reverts to his old habit of satiating his hunger here, and John sees him bite himself again, this time as a form of self-harm and what Arthur thinks is due punishment for becoming a monster, and they have their heart to heart about reaching bedrock.
John definitely knows something is up at this point, and is quick to reassure Arthur that they'll kill Larson, but they need to save the people of Addison first, if only to prove to Arthur that he's not the monster that either of them thought he was. John also comes to the realization that he might've been the cause for all of this; he remembers how the King said that knowing his love had scarred Arthur permanently.
So, after freeing Addison, John's main focus becomes separating them again, but this time it's for Arthur's own sanity. They do learn about the order of the fallen star, which John thinks is a better lead for separating them than searching for Anna Stanzyck.
That leaves us where most of my drawings of Dumpster Arthur are set timeline wise! They're in New York, grappling with Arthur's growing hunger and self-harm habits and finding a way to separate them, something Arthur is getting less and less willing to do, given as John is pretty much the only thing holding him together now.
I'm working on a comic about them finding a vessel for John, the construct body built by a fringe cult worshiping Hastur. This essentially is just an anchor for John, while he's still bound to Arthur, that body does give him autonomy and allows him to project without causing Arthur any stress (beyond emotional).
All of this is kind of subject to change, but a couple people in the tags of some of my art pointed out how interesting it is to focus on Arthur becoming more of a monster, something I definitely am going to explore a bit :^)
As a reward for finishing reading my very long brain vomit dump, have some fun(ish) Dumpster Arthur facts!
he wears a trench coat way too big for him because he thinks it makes him look bigger and more intimidating. john does not have the heart to tell him it just makes him look like a really sad wet cat
john takes on a bit more of a caretaker role for arthur when they reach new york! part of learning his own humanity comes with caring for someone else the way Lily cared for them in the hospital.
arthur also does have some nerve damage at this point in his right arm, so he does kind of twitch and shiver like a chihuahua when it gets particularly bad
john and arthur create a version of asl meant for one-handed signing, though it's rather hodge-podge to anyone who signs traditional asl. this allows john to communicate without arthur translating (noel has a bit of trouble reading the signs at first but learns quick) and also allows arthur to communicate silently with john.
mintyeggs. tumblr. com/post/752486252425740288/bourgeois-beware-arthur-lesters-fists-are-here-to?is_related_post=1 // this joke falls differently in a light of your recent lore dump. Arthur has standerts whome he is gonne to eat?
he hasn't eaten anyone since the prison pits!! he just really, really wants to. but I'm sure if he were just slightly more unhinged he might've taken a bite or two out of the freemasons treasurer
