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Personally, I would avoid telling the players "this is what you'll be fighting" unless they actively engage to extract that type of info. Players who make it to that level should have a healthy respect for Divination, Commune, and Contact Other Plane anyhow.
Personally, if your response to their divinations is quality enough and engaging enough, they will seek to do that more often so they can get relevant information as well as the color you provide to them. It's a fantastic way to download backstory into the game as well as give them some mechanical aid if it's relevant to their query
That said, if you want to foreshadow the tumultuous things, you can foreshadow a few things...
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The diviner sees Magnimar as it is now only to have the vision fade from it's current state to a crystal and metal city (with the same coastline so they know it's magnamar). Include a rebuilt Irespan stretching off to who knows where (we know where but it's vague enough to be glossed over but relevant enough that players will say "oh yeah, that was in the divination").
NOTE: This shows Xin's ambition but does not give away too much. Clever players might suss out that they need to head out that way but that's going to be painfully obvious when they rebuild the sihedron.
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Have the divining player standing in Dockway facing out to sea watching as a wave builds and builds until it towers over them. They see and feel it crash into them. They feel the sensation of gasping for air as they are swept away and in the chaos of that wave's wake, they see a woman's face screaming angrily at them and the sound of a wolf howling...
NOTE: This is a nebulous disaster but, again, kind of reinforces the direction the threat will come from. That puts the seed of a question in the player's mind which is, "how can I stop a city smashing catastrophe?" that they might not have thought about. The diviner might be alarmed and try to evacuate at which point the mayor dismisses the authentic vision as some kind of adventurer's PTSD (or other dismissal). You also foreshadow the Scylla but not in so direct a way as to make the players go "ok, gotta go get lawful and cold iron weapons now"... if they are well prepared as a party, they will have those options or, at a minimum, pay the costs associated with divining that kind of details. I know it seems picky but for "free visions" I like to leave things open to interpretation. Your players might look at the module cover art and know what the vision relates too anyhow but it's flavorful and thematic as a vision but also contains relevant detail that isn't scenario breaking.
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The diviner stands in a spotlight at the center of a dark, metallic room where a booming voice demands to know what the seven virtues of rule are (This is Xin's voice of course but players will not know that until they encounter it later). Attentive players will know the virtues and recite them back. However, they might not know it and it'll give them a tidbit to chew on. Eventually, the diviner will research/recall them and recite them back when asked (maybe over a period of a few days' worth of dreams). When he finally answers correctly, the vision changes. The player feels a nausea descend up on him and his sight wavers and distorts. A chorus of voices booms "WRONG!" Then, in succession, each runelord's face is illuminated (and described by you) and, instead, the seven sins the virtues were twisted into are recited. As the final of the seven sins is uttered aloud, a face in the shadows back lit by a brilliantly glowing sihedron rune roars, "ENOUGH," and the room is washed with fire.
NOTE: This downloads that the virtues of rule were corrupted. It brings the runelords (the iconic bad guys) into the frame again and the way that the vision plays out, PCs might interpret it as Xin in opposition to the runelord's visions (which is true) but it also puts that Xin has his own plans for those who would think they are in power (i.e. destruction). It can also be taken as a sign that the sihedron has the power to defeat the runelords (which is why the PCs underwent this in the first place) and is a reason why they should assemble it. If you don't give them this, they might just think "maybe we shouldn't put it back together" and then there goes your climactic encounter. They should want to assemble it.
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There are a ton of other things but basically use these dreams to download that juicy flavor text that PC's don't usually get instead of offering up the answers to "what are we going to be fighting" or other specific details.
You want to reward the players for being engaged and preparing themselves for this challenge. If they have made it this far, the probably are anyhow so don't just give it to them. :)