Everything Kayne does so specifically affects Arthur and John's relationship. It makes me think the Black Stone isn't the final goal. He wants the Day of Wrath. To do that he needs John and Arthur to be in very specific headspace in regards to their friendship.
He needs John to want them to stay together and he needs Arthur to want seperation. He's depending on John's fear of loneliness and need for control and Arthur's resentment of commitment (which Arthur sees as a loss of autonomy).
Like.
The Dagger: Kayne had to lure Arthur into the city. Like the black stone he can't see him, so Arthur had to come to him. He talks about the Patreon voting and who Arthur thinks will win (on a meta level). Then he gives Arthur the dagger. The dagger is a key component of the worst result for both of them (a tie). Arthur bleeding out and sent to Addison. John seeing Arthur is dying and going into the King. Then struggling and being cast out into the dark world. The coin which has two heads, lands on its side. Ties are bad and a tie means Kayne gets his hooks into both of them.
Yellow: Kayne specifically shows up to save Arthur and needs to save him for his plans to work, but Yellow does a bunch of things.
Arthur is forced to rely on someone he cannot trust. His personal autonomy is not returned with John gone, and worse, it cannot be trusted that Yellow won't try to steal his body. Unlike season 1 John Arthur knows where Yellow comes from and he remembers all the ways season one John burned him.
Yellow also provides a way to make Arthur not only miss John, but remember exactly how John used to be. It makes Arthur feel worse about who he's becoming without John. When John returns he feels guilty he has fallen so low. He's so thankful that John is back he actively avoids the red flags he see and is eager to show John that 1. Their friendship means a great deal. 2. That he trusts John perhaps more than he trusts himself right now. 3. He's sure John does not want his body and after all they've been through would never betray that fact.
Prime headspace to be manipulated.
🗡️
Kayne Keeps John In Dark World: he could have just sent him back to Arthur right away, but instead had him hit rock bottom. Had him do things for him. It reinforces to John how scared he is of the place. How alone he is there. How John falls back to being the King there without Arthur. Bonus Kayne uses it as leverage to get John to manipulate Arthur saying Arthur is too stubborn to do his bidding.
But then when all is revealed Kayne threatens them both to do his bidding to retrieve the black stone. He could have done that from their first meeting if he wanted to. He knows about Faroe. This is why I think it's not actually about the goal, it's about the journey.
Pops Matthews Head: needed to stop him from mentioning Kayne to Arthur, but also set dressing for later. Kayne points out Matthew had a choice in telling Arthur and yeah it got his head popped off, but he made the choice. John could have told Arthur about the deal and yes there would be consequences but he had a choice. (Also might be a nod to the Tie but I digress). John's choice is completely understandable up until the moment he asks for Arthur's memory erased (more on that later).
Shows up in the Mine while Arthur is Dying: John is on the cusp of losing Arthur again. Kayne reminds him what he has at stake. Not only is John terrified for Arthur, he also is terrified of going back to the Dark World, and this solidifies that 1: Arthur cannot die because John can't stand to be alone again. 2: Arthur cannot die because if he does his deal is broken and he's sent back. 3: Kayne establishes he isn't there to actually help John get things done. John cannot rely on divine intervention. John is soley responsible for keeping Arthur alive.
John stitches Arthur, but I'm convinced once he did Kayne just healed him up. He needs Arthur alive and he needs John desperate and guilty and he also needs John to become more and more attached to Arthur, not on a friendship level and not even on a survival level, but on a selfish level. Kayne wants John to feel entitled over Arthur and his body. Not just one or the other but both. He wants John to resent not getting a say about things that specifically effect Arthur. Kayne wants John in isolation to feel it keenly.
Get Arthur to New York: where there are people. For the first time Arthur not in places being affected by the King in Yellow's madness (because the King is indisposed due to Kayne. I think?). Arthur was bound to be reminded of how nice it is to interact with other people and John would feel isolated. Ramps up John feeling isolated after he just got back from the Dark World. Also causes a possessiveness of Arthur and pushes John to try to isolate Arthur in turn.
Makes sure Arthur Knows about the Deal Before John Can Tell Him: Arthur struggles with trusting John this season, but lands on John being on his side even if there's something wrong with his memory. He constantly reassures John that he is trusted. It's why they go to New York. Which makes John feel more guilty.
Arthur talks about how undefeated John is (John was super defeated and is guilty about it.)
John resents his reliance on Arthur and tries to manipulate him to establish his own autonomy. He has the excuse of the deal, but he's also making choices (like Matthew).
John resents his reliance but also saw what happened when they were seperated. Becomes angry when obstacles get between them (Oscar)
John will do whatever it takes to keep Arthur safe. Both because of his attachments and the threat of the Dark World. Thinks Kayne won't save him. Causes friction when he tries to get Arthur to give Oscar the stone. Gets him more frustrated when Arthur disagrees. John wants autonomy yes absolutely, but he also wants some control over Arthur's autonomy.
We also get a ton of indications that Arthur does not like being tied down/controlled. He resented being forced to marry Bella and misses the birth of his child because he has a crisis about it. His resentment of religion and Daniel's belief in it. Avoiding any relationship with Daniel because of the control he tried to exert wanting him to move to New York. He admits he felt a terrible kind of relief after Faroe died, not having to take care of a child anymore. He doesn't like when he is forced to give up his own autonomy. Even for the sake of others. And he knows it's shitty and he feels guilty about it, but something inside him struggles.
The Grey Stone: sets them up for the mission with the black stone so that Arthur would be keyed into things about the Order, but ultimately was just a end point. Kayne's real goal here was to finally reveal John was working for him. Arthur never even touches it.
Interestingly, although not necessarily Kayne's machinations, Arthur loses all the people he's met since coming back to their world. Larson and Yellow are banished. The Butcher is killed. Noel is bleeding out and sent to maybe Spain. Arthur writes down to forget Oscar (which might not be literal but an underline of Arthur choosing John). Cannot write Marie. I think Daniel is the only exception, but it's established how dangerous Arthur being near him is with Daniel getting hurt. Arthur cares about him but once again doesn't want to be tied to him.
So even if he got back to New York the majority of Arthur's ties would be gone. Just like how he has nothing really to return to in Arkham with Parker dead. All he has is John. So England! Right? Start fresh. No ties (pun unintended).
Yorick: Kayne specifically mentions the tooth. Now that the two are alone again Yorick can cause some conflict even if Yorick himself is on their side. Arthur sees a tool that can help them and John sees a danger to Arthur. Also John gets jealous when Arthur trusts Yorick over him.
Kayne Offers to Wipe Arthur's memory: reinforces to Arthur that John actually shouldn't be trusted when it comes to his autonomy when push comes to shove.
Arthur has been struggling all season to try to accomodate John's isolation. He can see the situation is unfair to both of them. He gives up on a friendship with Oscar, but also tries to establish an openness with Noel so that John has a little bit of a voice outside his head and Arthur doesn't have to be isolated from other humans. Arthur is happy to give John autonomy, but not at the cost of his own because, as established, he doesn't like sacrificing his autonomy for other people. He is being forced to with the John situation and because on his journey with John he has become much more self aware. He is trying his best to compromise.
But now the shoe is on the other foot, and when John is offered a chance to take Arthur's autonomy he immediately tries to takes it. No compromise.
Arthur thinks John fucked up on a human level because of the stress of the deal and his resentment for his lack of control. I don't think he clocks how afraid John is of being alone and that part of the reason he tries to control Arthur is so that he doesn't lose Arthur (Which pushes Arthur away because he does not want to be controlled. Not by Yellow, Larson, the Mine Creature, or John).
John on the other hand understands it was a shitty thing to do. Knows theatening Arthur's body was over the line, but doesn't see how attached to autonomy Arthur is. Arthur gives John an ultimatum. Either John takes over or he doesn't because that has ALWAYS been what it's been about for Arthur. John's betrayal reestablishes it firmly. Either they seperate or John takes over. He does not see or even want a middle ground. Middle ground was Yellow. Middle ground was Oscar. Middle ground was John working for Kayne.
Kayne Ignores John's calls when Arthur is Killed by the Hag: could be he just can't hear them, or could be that he REALLY wants John's fear of being alone to reach a max. Like as a star wars fan it feels like Kayne is Anakin Skywalkering John into fear of being alone so deep, and clinging on to Arthur so tightly that later on he's going to do something extremely ill advised to keep Arthur bound to him, not out of love but out of fear. Something that Arthur does not want
At the same time Kayne is also trying encourage Arthur's own selfishness about his autonomy and being tied down by establishing very complicated trust issues in Arthur where he cares about John and trusts him up to a point but cannot trust him with his body.
So that Arthur keeps forgiving John until a big enough betrayal makes it impossible.
So Arthur very explicitly wants them to have seperate bodies and thinks this will fix things.
And John does not want them to be seperated at all.
The resulting blow out results is maybe the Day of Wrath. John does a thing to keep Arthur, Arthur feels betrayed, spirals about how he could never trust John and does something to counter what John did. Arthur is stubborn you can't make him do anything, but you can ensure rebellion. Like, bonus points if John's actions seem to be in line with Kayne and Arthur thinks he's screwing both of them and freeing himself entirely from being controlled by others.
Whatever Arthur does sets off is exactly what Kayne wanted in the first place. The Black Stone was a bluff or just a key to getting Arthur to do the thing.
In conclusion Kayne isn't just trying to point them in a certain direction, he's very deliberately trying to manipulate how they feel about each other.
The real big bad was the friends we made along the way.
Now John and Arthur might be able to avoid this with the power of hope and love and friendship, but I think Kayne is doing all he can to make them have incompatible goals. He even says it never works out when they seperate.
Hopefully John and Arthur love each other enough to forgive each other.