Forms

The theory of forms is a concept originally proposed by the real life philosopher Plato. See Wikipedia for more information about forms as a philosophical concept. For its usage in this campaign, read on.
A form is a basic, fundamental, idealized version of a thing. In philosophy, this could be either a physical object such as a chair or an abstract concept such as piety or passion. This is technically also the case in Prima, however forms that represent physical objects are not typically very interesting. That is to say, the idealized and fundamental notion of a chair does not have the same sort of real-world impact as something like piety or passion. When the form of a physical object appears in Prima, it manifests simply as an exceptional object of that type. When a conceptual form visits Prima, it affects everything in the The multiverse. For example, the form of piety might cause many new gods to arise, churches to form, and grant powers to the faithful. The form of passion, meanwhile, might spread vigor, enthusiasm, and love to every sapient being.
These forms come and go over time, of their own volition. They arrive in Prima as a result of intersections with the The World of Forms, and there is always at least one form present in The multiverse. Should the current form be destroyed in some way, the entire multiverse shifts position in relation to the The World of Forms in order to bring about the next form immediately. This change may or may not be noticeable to mortals, depending on its nature, but the gods are sure to take notice.
When a form, physical or conceptual, intersects with Prima it is given some concrete representation, known as an Avatar. Physical forms are typically a direct representation of the physical thing, such as a chair or a lion. Conceptual forms, however, are much less predictable. They might appear as a powerful magic item, a being with god-like powers, or even something as mundane as a tree or a river. These forms might have almost no effect on the mortal world, or they might change the very nature of reality itself.
The forms became public knowledge during The Age of Mortality.  The avatar at the time was a being from the Far Realms known as Iapetus, who had the power to turn any creature into a mortal being, including gods.  Meanwhile, the presence of Mortality itself caused the barriers between the realms of the living and dead to break down, making travel between planes easier.  While the city of Sigil did exist prior to this boundary breaking down, it was only due to Mortality itself that the many portals had opened within the city, connecting it to almost everywhere in existence at the same time.  It was also responsible for causing the gods to be all around more fragile beings than they used to be, a state of affairs which they have still not fully recovered from.  A great many heroes took part in saving the world from a far off demiplane known as Vigrid, slaying Iapetus and bringing The Age of Mortality to a close.
Unfortunately, now a new form has come... and it has yet to be identified.  Exactly what is the current age, and what cataclysm will it bring? 

Credit

I was first inspired to use this concept by a real play podcast called Dice Funk, which you can find on any traditional podcast platform, or on its Patreon page. Each season is its own isolated story, with a different cast of characters.