The Waypoint & Refugees
Overview
Dairruin and Silver Mara were the first two beings to cross the threshold of the Waypoint, heralding a drastic change to 'the way things are' on Pern --- and on all connected worlds. Unlike the Dragons of Pern, Mara does not require a lifebond to exist, and she was quick to correct assumptions about her nature - up until she was convinced otherwise.
The Waypoint itself appears able to change who and where it pulls from; but Mara herself has made some headway into directing the Waypoint to serve as a true doorway instead of a dumping ground. Species have largely come from Eien, but recent years have seen connections to other Nexus worlds.
What is the Waypoint?
The short answer is a gateway. What it actually is remains largely a mystery to characters IC. In inspiration terms, however, it is similar to the Wormhole seen in the movie Interstellar. Notably smaller in size, the Waypoint is large enough to admit a large dragon, but not a dragon the size of a skyscraper. You can absolutely see the other side through it, and the vision definitely changes over time.
The Wormhole is described in the movie, and by online contributors on astronomy.com, as follows:
"Imagine a sheet of paper to be 2-D space, then a line joining two points on the sheet of paper is the shortest distance possible to reach that point, but if due to some disturbance, the space is bent ( achieved by folding the sheet of paper), you can pierce a hole in the sheet after aligning the two points together. That is a 2-D wormhole (which is a circle indeed). So what happens when you consider a 3-D wormhole? It becomes a sphere.
Now, you can see the other end of a 2-D wormhole (which is effectively the other hole in the sheet visible from the first hole, and one can see beyond the hole in the other direction as well). Same happens with the 3-D spherical wormhole where you can see to the other side of the wormhole as well.
Now, the means to bend spacetime : Space time can be bent by having a very big mass placed inside the space time (read Einstien's General Relativity ). So, it is safe to assume that the wormhole is a space time disturbance created due to a massive object."
For our purposes, the 'mass' needed to bend spacetime is related to the War on Eien and the False Dragon God ascending to the immortal plane. Why it linked to Pern is anyone's guess, but it is definitely linked to between in some manner.
Who are the Refugees?
While the first dragon and 'rider' to cross the waypoint were not a true pair-bond as the Pernese would know it, Dragonriders and their bondmates did eventually find their way to modern Pern. Most seem to have common memory of a place called 'Fire Ridge Weyr' - a place that current maps note was overtaken by the jungles of the Southern Continent over a thousand years ago. With the prospect of returning to their old home a no-go, even if they wanted to do some major bushwhacking, these non-traditional dragonriders set about making a home for themselves in the North. It's taken some adjustments, but some even live with the regular residents of Coast Hold.
Also numbering among the ranks of dragonriders who emerged from the Waypoint are riders of Pernese dragons who found themselves lost in time. As a group, they generally don't remember entering the waypoint, just leaping between. As a result, they all share the same basic story; they and their dragons were frightened, with the resulting jump going very bad, very quickly. Perhaps it was luck - or providence - but the Waypoint appears to have connected to those lost travelers and brought them forward to the 14th Pass. Like the rest of the strange birds calling Coast Hold home, these Lost Pernese find themselves fitting in better with the other-worlders over the native Pernese of the 14th Pass. Among this group is a large contingent of riders hailing from the Isla Genetic Program Center; a alternate timeline Pern where genetic mutations have become normalized among Pernese dragons.
Where do they come from?
The primary world linked to the Waypoint is the war-torn planet of Eien. Other notable worlds include Velgarth, Icarus, Alyskyr and different timelines and AU versions of Pern itself.
What is the Nexus?
Straight from Shard herself on the Nexus Dragons DeviantArt group page: "The Nexus is a hub of worlds and dragon adoption agencies who focus on bettering members writing, roleplay, and drawing skills. Join up with a character, then participate in world-specific challenges and group-wide events. Join panel-judged contests, adopt dragon-like sentient creatures from clutches hosted by world-owners, and play through story-lines that span a galaxy.
We focus on encouraging creative writing, and support a community where quality and activity are rewarded with awesome adoptables and incentives to have your own character's story given a chance in the spotlight."