In the days before the Auromian Empire, philosophers theorised about the existance of other worlds - a polycosm. According to this idea, if space is infinite, then anything that could happen, must be happening somewhere. For every choice that a person could make, somewhere there would exist another version of that person who had made the opposite choice. The theory of a 'polycosm' was an esoteric thought experiment, before it was eventually quashed by church doctrine. If there were infinite worlds, would there be infinite heavens and hells? Infinite gods? The idea was denounced as heretical, and forgotten.
What no one knew was that the philosophers were correct.
Tellaia is only one of infinite worlds, each a step removed from one another. The closer that worlds are to one another, the more likely they are to assemble each other. No one in Tirenia is aware that these worlds exist: it would take extremely powerful magic to move between them. What effect this has on the metaphysics of each world is unknown and essentially unknowable. However, there have been travellers between the worlds that have entered Tirenia.
The 'disease' known as the Great Death is in fact a sentient being from another world. The first plague struck a month ago - this was by way of a preliminary exploration. It will soon return, and more powerfully. Those it infects who die are the lucky ones - it has planted its seed in the survivors, who will succumb to a living death as they join the plague hive-mind. Eventually, all that will be left is a dead world where all living things have been assimilated into the necro-plague, before it abandons them to absorb another world. Some doctors in Tirenia are aware that the plague has a magical origin, but its otherworldly, intelligent nature is still unknown to all save the turtle-folk. These travellers have swum on the streams between worlds to come to Tirenia, where they have managed to blend in as just another beast-folk race. They are few in number, but dedicated to using their holy magics to fight the Great Death wherever it should rear its head, so that no more worlds can be lost as their own was lost. If there are any other visitors from the Polycosm, wizards of incalculable power who have managed to step between worlds, they have so far kept a low profile.
Art: Image from 'The Book of the Queen.' Anonymous, c. 1410.