Færrin is a lonely celestial body, sharing its sphere only with the sun that revolves around it. Despite this, some "extraplanar" entities find their way onto the planet since Aurelia breached the firmament. These creatures don't come from beyond Færrin, however. Instead, these beings are here due to the Infinite Horizon.
As best arcanists can tell, Færrin is no longer a single planet but instead an infinite chain of Færrins with ephemeral connections forming between them. With relatively little preparation, one can "slip" to an adjacent Færrin -- traditionally, all one needs is a simple magic device called a slide; however, some can forego even that.
Moving to an adjacent world involves little actual movement. Travelers report a sense of extreme but momentary vertigo, but no acceleration in any direction can be measured. To consistently identify the "direction" along the horizon in which one is moving, travelers created new nomenclature for their journeys. The origin is Færrin prime, and the two directions are voidward and starward. If one travels voidward once from prime, they're in Void 1. If they do so 19 more times, they're in Void 20.
Perhaps disappointingly, the worlds adjacent to Færrin prime were empty. They appeared as the planet might have had no one ever come to live on it. The same was true for the next worlds and the worlds after them. In fact, it wasn't until Star 46,656 that any sentient life was discovered: a sprite that had itself traveled from further starward.
The fact that other worlds are easy to get to and relatively safe has had massive implications -- resource and land contention is a non-issue for most, as the same mine or field exists infinitely many times across the Horizon. Warfare has also changed, as stealth becomes a matter of transporting an army up and down the Horizon, with counter-strategies of building forts "vertically" across multiple worlds at once.
Since the Infinite Horizon was discovered, adventurers have struck out to map the various worlds that spread out from Færrin prime. Most worlds are utterly devoid of sentient life, but some worlds are unique, with whole alien civilizations or strange natural phenomena. Exploration has been momentarily halted in both directions by so-called "walls" -- travelers who have slipped into Star 1,679,616 and Void 1,953,125 have never returned.
Methods for traversing the Horizon faster have been found or developed over time. "Chutes," where a slide might skip a few worlds, appear randomly. Some powerful spellcasters can use magic to teleport right to their destination. Finally, some vehicles called Sliders come equipped with a slide and can make rapid slips without intervention.