Planer convergence
the elemental planes merge and split and merge again. There are times when they only brush against each other.
When an elemental plane simply brushes against the opposing elemental plane it creates a wide spread extinction event that ripples across the planes, but full convergences of all four elemental planes means the complete destruction of all the planes. At full convergences not even the matter of the planes survives.
To defend against the total loss of life in each catastrophic convergence. The multi-dimensional being THOON has been tasked with preserving the species in a “Noah’s ark” situation. The process collects a viable specimen of every species. The mental effects of this process are less pronounced upon the predator prey species due to the instinctual drives. But the more evolved civilized beings lose most of their higher brain function for 1 to 200 generations. much of this degradation of higher mental function was mitigated since the construction of the nexus.
Due to an epiphany that a think tank of mages had they too for saw the coming calamity, they sought out THOON to learn from it. Soon after that the mages crafted the planer nexus. Due to the alien-ness of THOON the truth of THOON is kept from any that are unable to survive the knowledge.
The planer nexus is a non-dimensional “black box” of sorts. It lies at the center of the entire cosmos in the eye of the storm so to speak. From its observatories the planer guides can observe the movement of the planes and predict convergences and divergences. But also due to its unique location it also serves as a bridge between all the planes, and versions of planes. It is also a location that can be used to shelter in place to ride out hazardous planer phenomena.
The nexus is an artificial metallic “space station” with no resources of its own, all resources must be gathered and stored, ensuring that there will be enough to last through any disaster.
the party is approached by one of the planer guides. They are informed if the higher functions of the nexus, that it is not just a connection of planer trade. but is infact an enormous ark.
mass effect:
...what do you mean the citadel is a massive mass relay...
the party will be tasked initially with gathering resources to prepare to survive the coming catastrophe. thier success or failure will gain or lose them "nexus points" which represent the nexuses ability to preserve the planes and the life that lives on it.