So, there are a few things about Jevil I don't really see people mention, including his line, "THIS BODY CANNOT BE KILLED," but there's also Jevil's prison.
You're unable to explore this prison, only taking a fixed place throughout the cutscene and the battle. Once you win, the prison becomes inaccessible and the door disappears.
What if the door is like the teleport/Sans doors? Those were able to appear from nowhere, even when they weren’t being worked on. So they could easily disappear too (or possibly the transition to the "Man behind the Tree" rooms). The door clearly isn’t a normal door, because you don’t unlock it normally. You hold the key in front of it and it creates a mini-door inside the original. Maybe the mini-door leads teleports you to Jevil’s prison and the big door would just take you to a physical space.
Note: in both chapters, you’re never allowed to travel more than a few steps east of the Fountain, almost as if the world ends there. This corresponds to where Jevil’s prison would be, slightly east of the Fountain.
What if Jevil’s prison is actually an expansive void past the revolving platform, and you’re ‘teleported’ to it? Jevil’s claims about being the “ONLY FREE ONE” might not be as deluded as it seems. He’s ‘out of bounds.’ He’s in a Void similar to the one in the UT fandom. Obviously, his line “I CAN DO ANYTHING!” represents his role as a wild card joker, but it could have a hidden meaning. If he was free in this void with no consequences, he could be somewhat of an Ink!Sans-like character, able to do what he likes, as shown to him by the ‘Strange someone’ (which might be Seam himself).
He's also aware his whole world is just a game, as someone might see from an ‘out of bounds’ perspective.
One more thing to mention is how Rouxls relates to him and how he might be more evidence Jevil’s prison is a void (everyone seems to be talking about him right now anyway).
I think RK is just supposed to be a joke, but for theory purposes, he actually has two links to Jevil.
1. One of his hobbies is “Longe walks in the dungeon.” It’s a random line which isn’t really funny, yet is isolated, and RK winks when he says it. That wouldn’t be much on its own, but how can you have long walks in such a tiny dungeon? Either he just goes round and round in circles in the place where the animals and the three former kings were kept OR he visits Jevil’s prison, and has ‘long walks’ in the void beyond.
2. When Lancer talks about ‘Lesser Dad’ in Queen’s mansion while solving the puzzle, and calls him a ‘Puzzle pagliaccio.” (I think that’s how it was spelt). He specifies this, with a sad face, that he’s “more like a sad clown”. Granted, Lancer calls lots of people ‘clowns’, even ‘CLOWNS around TOWN,’ but again, it’s a very odd line which no-one (I’ve seen) can see the joke in.
I’m not really too certain of this theory, but there are a few pieces of evidence for it, and there’s one more thing to note.
Who is it that locks Jevil in this possibly otherworldly prison?
Seam.