Otherworlders

Otherworlders are a group of two species from outside the world of Caldonia -- the angels and the demons . They arrived roughly four centuries ago through a dimensional rift.

History

First Contact

"It would be a great boon if you would feed me."
"Will it hurt?"
"You won't even feel it. It's harmless."

— Eziareal Yehosephat's last conversation, with a demon he had just met.

Eziareal Yehosephat was a zealous Nogibozan preaching to anyone who would listen, alternating between sermons on fire and brimstone or paradise, depending on what he thought people would listen to. Not finding a great deal of success with building a congregation that would sustain him, he took a group of about two dozen of his most ardent followers into the wilderness "to find the gods."
In the forest of Chikal at the base of the Ridgeback Mountains -- an area covered with boulders wherever trees do not grow -- Eziareal's followers stumbled on a group of jahannan who had recently been transported through rifts into Caldonia. Convinced this was the arrival of the gods he had sought, Eziareal was only too happy to offer them anything they asked for -- in this case, food.
Being new to the world, the jahannan were not aware that taking life force from humans would be fatal without the most precision and care. Eziareal didn't even know what was going on when he fell to the ground, a husk devoid of anything remotely resembling biological processes.
Ezieareal's followers fled, sharing the story of encountering giant winged people who devoured souls. Naturally, they called the new monsters demons, evoking monsters of the underworld from Nogibozan dogma.

War

"Ugh. The humans are coming again. They've got a ballista. "
"Any structs?"
"Doesn't look like it."
"Ugh. So tedious."

Calling the ensuing conflict a war is probably over stating things, but it was definitely bloody. Small militias gathered together to hunt down the demons, unilaterally failing, often with staggering losses.
It should be noted that there is no verifiable record showing that the jahannan ever forcibly fed on sapient Caldonians after the first contact, but plenty of stories spread through human camps and communities, many of which are still believed to this day. It is also curious to note that there are no records at all indicating jahannan ever initiated a violent conflict.
During this period, the population of jahannan in Caldonia grew, from the few Eziareal met to scores of them in a few years.

Angelic Visitation

"No. We would prefer not to."

— Janus Manstarkin, when asked if he would encourage his fellow sera-im to help humans fight jahannan.

A few years after the first contact with jahannan, the remainder of a raiding party of humans, wandering blindly through the forest, their only goal putting as much mileage between themselves and the demons, stumbled upon a small camp of what turned out to be three sera-im. The sera-im gave some food to the humans and tended their wounds, which involved some very minor sorcery. This sealed the image of sera-im that humans have carried in the intervening centuries -- if their conflict was with demons, surely these were angels.
No sera-im ever fought in the conflict, deciding to observe until the final result could be seen and the rules of human-otherworlder relationships could be determined and codified. But humans vastly preferred a supposedly beatific -- if hesitant and intermittent -- trading partner to the more frightening demons they chose to try and drive from their world.

Detente

Time to repurpose the weapon forges. Humans just aren't willing to run into certain death anymore.

— Mintella Shrikefeather, elven entrepreneur.

Over the following two-and-a-half centuries, the frequency of human-demon skirmishes oscillated wildly. Some charismatic but bloodthirsty human leaders would organize attacks several times a year. Other times a decade would pass between battles. Humans had the most success when they were able to use structs in the battle, but they could never bring any other allies to bear -- they did not trust their feren slaves to fight, and cerebrek could not be convinced it was a worthy cause. Elves were more than happy to administer raids and attacks if there was any gain in it, but never raised arms in support of either side.
Cooler human heads eventually prevailed. And while there was never any treaty signed, Human aggression against demons ended for a half century, until demons agreed to help feren in their war of emancipation.
Otherworlder presence in Caldonia grew during the centuries of conflict. Several thousand at least of both sera-im and jahannan lived in Caldonia. However, both types of otherworlder emphatically claim that there is no longer any gate to their reality of origin. They cannot leave Caldonia, and no new otherworlders can come.