History of the entire world I guess
Industrial Age (Ante Calamitas)
An Abyss Gate is an airlock whose purpose is to allow submarine vessels to pass the world dome into the Abyss.
It has been known for thousands of years that the world is largely flat with an energy firmament covering the entire world like a dome. It wasn't until around 200 years ago that scientists discovered that the dome had a resonant frequency that allowed matter to pass through out into the beyond. At first only microscopic particles, but through advancement in technology, larger and larger things could be sent through the dome. Eventually scientists could send small, mechanised probes through. These tiny submarines were meant to gather data of the water-filled void outside.
It was eventually also discovered that the dome could be phased out with proper application of frequency and amplitude of the resonant frequency. This, of course, was an inherently perilous thing to do in case the dome would remain phased. While a small hole would take millenia to drown the world, the rising sea levels would eventually wipe out civilisation as we know it. Thankfully, the dome rematerialized as soon as the resonator was turned off and the Abyss Gate technology was born.
Purpose and function
The Abyss Gate allows for a single submarine to be lifted into the airlock and suspended in its center. The dome inside the airlock is then partially phased out by a massive subharmonic resonator, allowing water to pass through the dome and fill the airlock. The process was initially done by simply phasing the dome completely out of existance, but the sudden water flow could lead to damage to submarines, docks and even injuries of passengers when the sub was tossed around by the massive amounts of water. The upgraded resonators can phase the dome on a sliding scale, rather than all or nothing, allowing for a much safer filling of the airlock. Once the airlock is filled, the dome is fully phased and the submarine can leave the dome and enter the Abyss.
Only a single gate has ever been built. It is constructed in the far east on a respectable distance from the apsis of Allír. Building one of these monstrosities is an extraordinarily expensive and resource intensive task. The gate was built before The Cataclysm and due to its remote location, was largely unharmed by the disaster and can still be used.
The gate has a rigorous security system built into it to prevent the airlock from ever being open at the same time the resonator is active. If the airlock is open, the resonator has no power and is completely cut off from the system and cannot run.
In order to book passage through the dome one must file a navigation plan with date of departure, route and date of return with the Communal Authority. The gates are maintained and operated by Egress Incorporated that also handles and receives all travel fees. It is an expensive prospect to travel through the gates and Egress Inc is making a lot of money on scientific and corporate travel.
It's a connected network of mist-gates (or modern tech variations of such) that allows a person (or vehicle, for the large ones) to "slingshot" through the Veil, passing into the Shadow Realm and back again, appearing at a desired Slinggate in Dunia, thus allowing near instantaneous travel anywhere in the world.
Since the entire Kykr civilisation was based upon the Slinggate Network, they had no need for large, long-distance transportation vehicles and methods. No world-spanning rail network, no significant air traffic, no long-distance lorries. Since all vehicles were essentially shade-powered electrical vehicles, they didn't have the capability of long-distance travel anywhere.
Once the Cataclysm hit, the Slinggate Network was disabled. There was not enough Shade to power the gates, and no Shadow Realm through which to slingshot. The Kykr civilisation was stranded, incapable of going anywhere.
Fusion City and the Slinggates
Fusion City was a brag-build of science. Basically a bunch of rich assholes wanting to show off. They made an entire megacity that worked independent of the Shade-network and they didn't allow anything shade-powered within the city. Even the local slinggate was powered by a dedicated fusion power plant. The slinggate in D'jatlha was turned into a major transportation hub when Fusion City was built around it. It was converted to run on electricity, rather than pure shade, as part of the "only fusion" thing.
When the Cataclysm hit, the gate acted like a funnel, just like any other slinggate or shade powerplant, wiping out the vast majority of the "Old Town" district in Fusion city, scattering rubble and corruption throughout the neighboring district. The residents of Fusion City were cut off from the rest of the world, unable to travel anywhere but into the Wastelands.