“Everyone is, of course, familiar with the day-to-day existence of the planet we call Earth. Rocks (and rockets) move according to Newton's laws, the sun rises, plants grow, people cheat, and nothing is certain except death and taxes… yadda, yadda, yadda, etcetera, etcetera, etcetera. If we extend the name 'Earth' to our universe (for a purely linguistically convenient convention I hope you'll understand, I am not one of these fools who thinks that the Earth is literally all there is. What nonsense!) then we discover the existence of another… eh, you could call it a 'universe', a 'plane', a 'dimension'. There is no really good word for it. What you discover is the existence of Aether, or so we call it. And this, is what fundamentally changes how we understand magic.”
— Transcript from a small part of a public lecture by Archibald Trout of the Slough Institute.
For all of recorded history, humanity across the globe has agreed that we are not alone. What we share this world with, however, is somewhat disputed; whether they are known as Devas, Kami, Jinn, or Angels it seemed unlikely that they referred to wholly different entities. We know now – mostly through a few interactions with these entities that inevitably turn out to be mostly unhelpful – that these beings come from another plane: one that humanity has most commonly dubbed Aether. People had theorised that such a plane had to exist previously, but whether there was only one plane or a plurality of them had been the subject of much debate for many years. Yet it is now understood that only one plane exists, even though two main groups of entities, generally known as Celestials, exist within it.
These factions 1) are most often called Angels and Demons in the west. They seem to have been diametrically opposed to each other, and each others actions, for as long as humans have had contact, but no consensus has been come to on what exactly their disagreement is. Some more blasé witches have simply come to conclusion that they are intrinsically and intuitively antagonistic, and have given the matter no more thought. Angels are often proud and principled, while Demons are mostly conniving and driven – though this does not describe every interaction perfectly (and those are surely only a small subset of all the Celestials out there). The problem with coming to any sort of coherent conclusion on Aether and Celestials is that very few texts written by humans have ever put down things in a simple, easy to understand, opting instead for grandiose metaphor that quickly loses context and meaning. The few ones that attempt to use simple and concise language are anything but, and end up circular gibberish.
Angels and Demons have also been seen to use magic on a very regular basis. At least, it appears to be magic to humans. It could be perfectly natural to Celestials but the effects have seemed to be exceptional to humans; these effects are exceptional even for witches. Where a witch could impose wards on a space or curse their foes to weakness, it is not unusual for Celestials to be seen providing shields of light and smiting their foes with glowing fire. No one really knows how Angels and Demons are able to produce such miraculous effects as they seem to be able to work with what could be called sneering indifference for the known disciplines of magic. It is, indeed, a mystery to everyone.
The oldest and most venerable method of interacting with Celestials is summoning. This simple, three syllable word is an umbrella for a diverse range of practices and techniques in attracting the attention of a Celestial and bringing them to your location. At this point, a witch often hopes for a bargain for power, information, or in some dire cases help with menial tasks. To aid with convincing, or convincing, a Celestial to help someone, it is common for an offering to take place as part of the summoning spell to give up tokens that a Celestial would favour; also wards and bindings are very commonly put in place to prevent the Demon one just summoned from up and walking away. Stories travel far and wide about the poor fools who manage to offend the Celestial they've just summoned, as well as mess up the circle of protection. Needless to say it is a good piece of advice to not be that guy. In truth, summoning is a delicate art, but not one that is too esoteric and in-depth that most witches can't pull one off with a little application.