Summoning in the Wikke Archipelago
Natural, Unnatural, and Supernatural: Summoning as a magical practice is by and large adjacent to ALL of the different magical systems and traditions developed in the island chain. Like most other magical processes, it deals with energy cycles, but where magical cycles are easy to experience and interact with directly, the processes that allow for summoning are a bit more obscure. Most knowledge about the field was accumulated by two different scholars, the first being Occultist Happ Wordden about two hundred years before Veck's Regency Era, who wrote a rather lengthy tome titled A Catalogue Of Visitations identifying the different entities that could enter the world via summoning. The other was an alchemist and astronomer named Nathanial Harker who wrote The Cacophony Of The Spheres seventy years later, a treatise on astrological cycles vital for the practice of alchemy and the limitations of working within those cycles, which is credited for inspiring the creation of the first Alchemical Orrery. This book included an appendix correlating multiple astral bodies and their orbits with the entities described by Wordden decades before.
Spirits And The Netherworld: According to both Wordden and Harker, summoned entities fit into four broad categories. The first, most readily understood, and easiest to contact, are the spirits of the the departed, because they have an established connection to the world already. Through the careful questioning of these spirits, occultists and necromancers have gained vital insight into the cycles of life, death, and undeath, and have also made some headway in understanding the afterlife... or rather, afterlives, plural. When a person dies, they may awaken to find themselves in a dimly lit, shifting counterpart to the waking world which has variously been called the Netherworld, Underworld, or Afterworld. By definition, these are the spirits easiest to summon because they are already so close to the world of the living, for the same reason that it takes less time and effort to walk down the street to a neighbor than it does to travel across town.
What In Reincarnation?: In other, more difficult cases, spirits appear to reside in some sort of realm beyond the Netherworld, though exactly what the nature of this world is remains unclear. One notable case study documents a spirit who told the summoner, "I stand in a grand library, and can recite to you one sentence of all these books. Which would you choose?" before the ritual connection was lost. The most baffling outcome appears to support the idea of reincarnation, in that the spirit believes that they are alive and well if they can be communicated with at all; in the most notorious case a man who died in a construction accident claims he was lucky to get out of the way of the wall that crushed him. Scholars have debated the meaning and implications of this conversation for decades, with the prevailing theory being that the spirits are in denial about their state, but a vocal minority promotes a more radical idea; that rather than contacting the spirit of the departed in this world, the summoner managed to reach another universe entirely where events turned out differently and the counterpart to the deceased survived whatever killed them in the original universe. Nobody knows how to prove or disprove this theory, including its strongest supporters.
Demons Versus Archons: Spirits tend to be the concern of necromancers specifically, those who deal with life, death, and undeath, while summoners proper are more concerned with (and associated with) two mutually antithetical beings from Beyond. These come in a variety of shapes, sizes, and forms, but follow two broad categories of similarity, those being Demons and Archons. Demons are physically, if not physiologically, associated with fire, sulfur, and high temperatures, and socially or mentally appear fixated on games of chance and skill. Archons, in contrast, physically appear to be made of ice or crystal, and are associated with extreme cold and frost, and are rigidly logical and organized. These extremes between hot and cold, chaos and order, make these two categories of entities mutually antagonistic, but at the same time means they are unlikely to ever meet each other without the intervention of a summoner in the first place; that said there are natural magical cycles that make it easier for one or the other to cross over into the physical world of the islands without help. Their goals and motivations are extremely difficult to discern at the best of times.
The Umbrals: The final and most obscure category of summoned entities are the Umbrals, beings who are either made of or perpetually cloaked in darkness. Of all the otherworldly entities they are the most difficult to contact, but arguably the easiest to work with once contact is established; while getting a Demon to do anything requires a professional gambler's dexterity and head for odds, and working with Archons is best done with the aid of a very good lawyer, Umbrals literally just trade favors. Simply put, do something for them and they will do something for you. Of course, what the Umbral asks might be something beyond the summoner's ability or skill, or take decades to successfully manage, or violate various legal, ethical, and moral codes; there isn't any clear correlation between the difficulty of what the summoner wants and the difficulty of what an Umbral wants in exchange. Significantly, Umbrals have been known to intervene in the world without the aid of summoning rituals during times of great strife, granting great power to someone on the verge of death and allowing them to turn the tide of battle; how and why they choose to interact with one specific person out of all those involved in the conflict, as well as what they get in exchange, is not understood. (Different Umbrals give different answers when questioned, so it may be that there is no overarching ethos at work and each individual Umbral is pursuing its own distinct goals.)