How does Grimdark work?

When Rose went Grimdark, it was because she was hearing the voices of the Horrorterrors after she prototyped the magic wands with the Grimdark book that talked about the Horrorterrors. It's worth noting, however, that before this point, the whole "grimoire" thing was just fake to her, and she didn't actually believe in the Horrorterrors. It's also just kind of an unlucky coincidence that this spooky goth book she got turned out to be an actual account of real creatures that float in the Furthest Ring. If Rose had prototyped something similar, like idk a book about Halloween or a book about Zombies, she might not have ever heard the Horrorterrors at all.

Rose is also a Seer and they're granted the power to "see" and hear things other players aren't privy to all of the time. Needless to say, she went Grimdark because she had the misfortune of turning a real book about real magical horrifying eldritch beings into some magic wands. Sburb Alchemy mechanics are kind of just weird but also OP because things that seem like they'd be useless end up being incredibly broken and super powerful. Rose makes the magic eldritch wands in a montage of her making other "silly" items too, so I doubt anyone, including her, would've seen the whole Grimdark thing coming.

They also refer to Jade as "Grimbark Jade" when she goes all screwey and evil, but this isn't necessarily because she's under Horrorterror mind control. The HIC took her over in that moment, and just made her momentarily evil. Calling her Grimbark was just kind of a pun bc her clothes were dark and she went "bad" like Rose did for a second. As far as I'm aware, characters only become "Grimdark" if they listen to the voices of the Horrorterrors for too long and do their bidding. Characters like Feferi aren't affected because her Lusus isn't a Horrorterror but an "emissary" of one? She seems to just be immune either because she's a Fuschia blood or some other explanation they just never really elaborated on, idk Feferi gets sidelined so hard for no reason other than there were 11 other trolls before her lmao so like there's barely any info as is T_T

Grimdark is among the things out there with a whole lot of speculation and not a lot of examples to base that off of. As far as in-canon explanations go, we get "the fabled blackdeath trance of the woegothics" and "completely off the deep end in every way" - neither of which are very specific, but we work with what we got. How you regard that also entirely depends on whether or not you regard Grimbark to be worthily considered, as well as whether or not you believe Eridan went grimdark during Murderstuck.

As far as Rose and Jade go, they're thankfully described pretty consistently. We see Rose "go off the deep end" and fixate on the one goal of revenge; whereas Jade is less entirely/strictly just "under the control" of the HIC so much as she's having her "inhibitions" let loose. On top of that, Rose has pretty clear, direct connections towards the Horrorterrors that lend a way for her to become influenced by then; whereas Jade is being double-mind-controlled by the Condesce, whose lusus - Gl'bgolyb - was confirmed in a book commentary to have been an actual horrorterror brought to Alternia by Doc Scratch. While less direct, you could still use that to argue that a connection to Horrorterrors existed, given HIC has contact with and shares a mental connection with said horrorterror.

Based on that, I'd personally point to those few factors, albeit still pretty vaguely.

1. The transformation you undergo is that of eldritch influence.

2. It marks a loss of control or inhibitions, specifically in a bleak sense (as to contrast the loss of these seen in its inverse of Trickster Mode, and hence its spoken tongue being referred to as "broodfester")

I've also seen a handful of talk out there about how it relates to your aspect, and how it can be indicative of inversion, or over-embracing it, or undervalue it, etc. but I personally struggle to find worthy evidence of this. While it's true that Rose going Grimdark is especially "Voidy" - I think it's better interpreted, or at least more narrative satisfying, from a personal perspective than one of game mechanics. Rather than Grimdark being what happens when you are "heavily resistant to their trUe calling" or "corrUpted in some way by an oUtside influence" - I prefer to see Grimdark as specifically something that occurred when Rose Lalonde rejected her calling as a Hero of Light - as Grimdark with all it's voidiness, obfuscation, and what have you is what would be Rose's way of manifesting powers in defiance to her aspect. What you define to be "in defiance with your aspect" should be caught under a much broader umbrella than Grimdark, and up to each person to decide best fits their OC and how they engage with their role.

I think the most important takeaway you can have though isn't from canon at all. In the end, we know JACK SHIT, but Grimdark still sits there as a very powerful narrative tool just waiting for its potential to be sapped. If you end up having an idea for what you want to do, and you worry that it may not align well with some invisible, unspecified guideline on how Grimdark works, remember that telling a good story will always be more satisfying in the end than "being accurate." I think a lot about how Grimdark works because I find it fun and interesting, but when it comes to OCs or AUs, I sort of go "to hell with it" because you'll get the most out of making Grimdarkness something PERSONAL to your OC as far as why they're driven to such a dark and volatile point, what it means to them, and what you intend to accomplish with it.

I think the one really crucial thing to understand about "grimdarkness" is that it is a joke transformation. To describe a work of fiction as "grimdark" is to playfully or even derisively point out that it is overly violent, depressing, or both - edgy for edginess' sake. Rose's "grimdark" form is a parody of that kind of storytelling, the same way the horrorterrors are a pastiche of eldritch horror rather than an attempt to play Lovecraft's signature style straight.

To this end, the way that grimdark "works" is for the most part superficial. Rose is cloaked in a storm of darkness, which obscures her from viewing screens and might cause her observers to worry about her, and speaks in a strange and alien way which other characters find difficult to understand - but her personality remains fundamentally the same, and she doesn't really gain any completely new magical abilities. Being a "form" unique to Rose, it reflects Rose's very particular mental state at the time; having had her understanding of reality and the horrorterrors filled in its head, and then discovering her mother's death, she goes on the warpath in the only way a repressed, angsting teen girl knows how, and the overly melodramatic and excessively gothic effects are the manifestation of that.

There's a few different ways to look at it.

In essence, it's Rose having a manic episode. She's stopped caring about the big picture, and went no contact with everybody in order to focus on her destructive vices. Kind of like what her Mom did around the house. When Rose meets up with John, she's not possessed or brainwashed or w/e. But she is kind of embarrassed to be seen like this.

Textually, Rose is getting info from two sides by the time Grimdark happens: The Horrorterrors and Scratch. They're opposed to each other for reasons that we don't know this far into the story. I posit that Grimdarkness is something that was bestowed upon Rose in an attempt to break her connection to Scratch. Not out of malevolence, but perfectly timed to appear as such.

I greatly appreciate everyones replies!! I've got a lot to think about now. Seriously, thank you all. It's really nice to ask these sort of questions on here.