Interesting as it could be to go full-on chronological order, I think it works best to start after PLA canon and a reunion. I've written enough of them anyway, people have to be getting bored.
Our basic premise is that, while the twins aren't properly telepathic, there's a tie there that allowed Fennel's work re: the Dream World to help with Emmet's search.
The intention was to use it to get in contact, but that didn't quite work out. What actually happened was that, when Emmet was asleep, he got a glimpse into whatever Ingo was doing, and presumably vise-versa.
Long story short, the information gleaned from this was able to push him in the right direction, and once he knew to focus on Hisui, things worked out.
Maybe I'll go into more detail later, but right now, that's all we need to know.
Where we pick up, they've gotten safely home, but things are still weird on the dream front.
(I'd say it would have been in Hisui, too, but Cresselia was hanging around Coronet, so let's just assume she (or Darkrai, I guess) passively overrode what was going on.)
It's not super obvious at first. Emmet is our POV character for this, so, to him, he things the Hisui-centric dreams he keeps having are just his subconscious processing everything he's seen and learned over the past X amount of time.
(That's an important detail here: sleep plays a big role in processing information and emotion. This setup hasn't actually let either twin rest, being more like dozing than sleeping.)
(It's just weirdly exhausting, considering a healthy amount of sleep is involved.)
Anyway, in the moment, it makes sense that he's finally just getting to process everything he learned and saw, be that firsthand when he was in Hisui, or through his connection to Ingo.
And, to be entirely fair, there are other subject matters involved, usually from the Unovan side of things. There's really no reason to jump to conclusions yet.
The point at which things start to seem strange is less a dream, and more a nightmare.
The focal point is actually an absence of anything. In this dream state, he doesn't know where he is or why and, with further introspection, has no idea who he is. There's a sense of wrongness, but he doesn't know what it pertains to, just that something's horribly amiss.
Second point of interest-- somehow-- is that he's caught in a blizzard.
That's honestly so far from his focus that it really is an afterthought, and may actually be what he wakes up on.
The big thing really is just that creeping sense of dread/wrongness and lack of identity.
That's... very off, but still in the realm of possibility. He'd seen bits of the Alabaster Icelands; his unconscious mind could have cobbled that together from circumstance.
X nights later, it's followed by a very similar setting, and while his attention isn't brought to the whole 'identity crisis' thing, it's still very much an existing factor. It's more an accepted fact in this one, though, while other things are happening.
One point of interest is the sensory information: numbness in the extremities and how little any attempt to huddle up is helping, how utterly miserable the cold air makes it to even breathe, the dogged determination to keep moving forward, even though it's accomplishing less as time goes on and exhaustion sets in/the snow builds higher up.
The second point is the shapes forming at the edge of his vision. They're hard to make out-- they mostly blend in with the snow-- but he knows there's something there, and he thinks it's what he was missing before. If he can catch up, he'll be okay.
He catches a glimpse of it again and tries to call out, but only manages the first half before his voice breaks. Keeps going but, eventually, tires out and collapses.
There's that sense that he needs to catch up, that he'll be able to help and everything will be alright, but he can't hear him, and he's physically unable to go on.
The shape grows fainter and fainter, and has all but vanished when he wakes up.
Emmet's, uh. Not okay upon waking up. It wasn't his voice that called out, and the first syllable was telling. The thing that blended in with the blizzard was white. And, when he collapsed and got a good look at anything on him-- the arm under his head-- it was covered w/ a black sleeve.
All Emmet was ever told about Ingo's appearance in Hisui was that he woke up without any memory and was taken in by the Pearl Clan shortly thereafter. Nothing about it would have prompted his subconscious to put that together, let along in that level of detail.
And, right now, this... feels familiar. Not like coming down from a nightmare, but like waking up after a look into Hisui when things were going very badly. There's a very real sense of fear, specifically for his brother.
It's ungodly early, but based on what he's pretty sure just happened, there's a good chance Ingo's awake, too, so he takes a minute to collect himself and goes to knock on his door.
There's a hesitation-- more puzzlement than anything-- but he's given the go-ahead and, upon stepping in, it's pretty clear Ingo didn't just wake up. Chandelure's hovering nearby, upset, and he was caught midway through wrangling Gliscor as it tried to get in his face.
Ingo takes one look at him and asks if he's alright, only to get a question in return-- did he dream about being lost in the blizzard, too?
He freezes. Takes a second to consider it and then, confused, asks when he'd mentioned that.
He didn't, but that's what he woke up from, right?
...yes? How does Emmet know?
He just watched it play out, too.
(It only makes sense that, given the reciprocal nature of this, once Emmet went ahead with the dream-sharing thing, Ingo started seeing glimpses of modern unova, too, w/o context.)
Emmet uses that as a point of reference. Obviously something's gone wrong, there. Or it's working as intended. The way things actually panned out wasn't the plan, after all.
There's a bit of a lull as the implications sink in. Emmet asks if he wants to talk about it.
For his part, the question seems to come as a surprise to Ingo, who'd spaced out a little.
Says he's not sure what that would accomplish. It happened, and he didn't really understand why for a long time. While it makes more sense now... what is there to say?
Sure, they could debate whether or not there really was anything there-- if it was a Zoroark or a trick of the blizzard-- but it kind of seems like a waste of time.
Emmet's gearing up to say that's not really the point-- the important thing about it was the fear and loneliness, not whatever the reality was-- when the topic turns on him.
Turns out Ingo had been taken off guard because his thoughts defaulted to a few nights earlier, when he'd had a dream of running, panicked, through familiar-unfamiliar tunnels.
He'd kind of just passed it off as a stress thing combined with very faint memories, but in lieu of this discovery, it seems a little more important.
Did... did Emmet want to talk about something, then?
And oh. Yeah. That did happen the other night, didn't it?