The Spell Plague is outright ignored by Haze, anything that happened as a result of the Spell Plague in 1385 DR, can be widely ignored, and assume that the timeline in Haze branched from the regular forgotten realms at this time.
In player-occupied settlements you will encounter various factions and degrees of self-rule. Grouping together is key to survival.
Any magical classes coming from Toril will experience that, while the use of magic is still possible, it seems to require more effort than before. Anyone with a divine connection will also find it more 'distant' than what they are used to, though not severed, and anyone with a connection to nature will encounter a vibrant natural world, though in places not free of sickness or taint.
Honestly, not much. Narumer is not something your character will have ever heard of, know anything about, or even be intentionally heading towards. As long as you were on a ship, sailing in the ocean, at the time your character is created and ventures into the game world, the rest will take care of itself.
Nobody knows for sure, so many people have crashed here, and all have different stories and ideas about where they were heading when they crashed/arrived on Narumer. Nobody came voluntarily, to your knowledge. As long as your character is from the world of Toril, and is not using an unsupported race/subrace, you will find you fit right in.
Once the veil to Narumer is crossed and a character washes upon the sandy beach of the Island, they will find their magical connexion to be different. This ancient island is not governed nor affected by The Weave the same way their land of origin may be. Here, magic is a fickle thing, old and exhausted. It is a finite energy, and with each conjuring, each miracle and each pact called upon, this energy trickles and fades further and further. This remains true for each source of magic, be it the structured and precise formulas from a Wizard’s grimoire, the raw manifestation of a sorcerer’s bloodline, the plead of a faithful Priest being granted by the divine, or the soothing harmony behind a Bard’s melody. Magic is fading.
Prayers, although distant and muffled, are still answered by those with the gift to call for it. Priests of various walks of life will know that something is indeed different. Yes – Their patron god still answers their prayers, but it does not feel as it might have before. Granted miracles bears a distinct taste, and rings of a specific note – They all remind you of the island itself, the way the sun shines on the sandy shores, the smell of salt and seaweed that permeates the air, and a fleeting dread that in time, the source of it all will inevitably falter and wither away…
Religion is as diverse as there are believers. While characters may be used to exceptionally rigid precepts and domains, with churches encroaching on one another, The Divine on Narumer is not exempt from the withering of magic and in doing so, characters bound by the divine will realize that their patrons may have much less defined spheres of influence – as if will, intent and faith intertwined with one another, and spread thinly over all matters of life. A mist, or Haze, if you will…
In addition to completely ignoring the Spell Plague of 1385, players may expect severe differences in a Patron’s “Cannon Lore”, which for obvious reasons will not be disclosed openly. “Events” and “facts” from official Faerûn literature may be true, or not. Keep in mind that almost nothing is “common knowledge”, especially when regarding matters of faith.