The interesting pieces I gleaned from the 5 Nations on the subject are.. odd.
First, the Day of mourning may have initiated in a city that was known to be the 'Area 51' and birthplace of the first Cyrian Warforged. Now located buried in the sea of glass... which may be growing. {Couple this tidbit with kowledge of a certain Eldtrich Machine in one of the published modules
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Second, the Capitol city, far away from aforementioned epi-center, was rapidly subsumed by the grey-cloud, apparently being a secondary epi-center. Some survivors from across the nations boundry were witness to this. Some even claimed that the mists initially emanated from the Castle towers.
Third.. and most odd.. that the mists confined themselves *exactly* to the current dates political boundry. {Not sure if this is a writer not thinking clearly, but the same book mentions that Cyre's boundry lines were in a major state of flux, from day to day if not hour to hour.} Arcane magics do not have the capability to be this based in human society...
So.. my reading of the mystery is thus: Divine intervention into an arcane process that may have involved infusion of planar matter into an Edlritch Machine.. that is currently buried but still working, deep in the sea of glass. The arcane process was in its final phase and was being completed in conjunction by both the Magewrights of House Cannith and the royal Sorcerer {via a complicated ritual spell casting}. The planar matter is still seeping in through a not completely closed portal and will eventually spread across the entire face of Eberron!
That intervention could have been initiated by a number of factions, but the primary 'supernatural' and well known agents are the Blood of Vol and the Silver Flame, both of which have had, and still have, issue with Cyre.
The Lords of the Duct may have been involved in passing the correct, and appropriately adjusted, information to inspire this action.
As campaign information, the above 3 bits of info should be relatively common knowledge to those looking into the situation. As such, both the Church of the Silver Flame and Karnnath would have proclamations out ensuring thier innocence in the matter and pledging support to the discovery of the true cuplrit...whether they beleive these 3 facts to be true. Safer to protect against ignorance
eh.. or not. This is just one potential explanation.. altho one I will/am using in my game. The Delkyr want access to the Eldtrich Machine as it may be able to open a portal to Xoriat as well.. thats my meta-plot.. and fortunately tied only to what the Delkyr think happened, so any 'revalation' by WOTC can't screw with my campaign