"What do you suppose that thing actually was, anyway?" you ask of the room in general. "Briar, you mentioned something about the Twilight Realm?"
"It probably wasn't from the Twilight Realm," she admits, "but something like it."
As she goes to work on Cordy, your fairy companion explains how, just as a large enough concentration of matter has the gravity necessary to curve or even warp space-time, a similarly large concentration of metaphysical forces can lead to comparable effects on other levels of reality. Simply by supporting life - and by association, death - a world casts "shadows" into those other layers of existence, in the form of the cycle of birth, life, death, afterlife, and rebirth. Once that life achieves enough sentience to recognize elements of the environment, and especially as that awareness advances towards actual sapience, those reflections begin to increase in number and complexity. By the time a functioning civilization emerges, entire new little worlds can be born, complete with their own strange metaphysical laws and inhabitants - which in some cases are one and the same. The process is only compounded once magic gets involved.
One of the more basic reflected worlds is Shadow. Everything that exists on the surface of a planet in orbit around a living star casts a shadow at some point, and life learns to recognize the phenomenon. Predators and prey alike use shadows to conceal themselves, and even simple plants respond to shadow - not infrequently by growing sickly, if the shade in question persists too long or is cast by a larger, stronger vegetable rival for sunlight and soil. Life knows that shadows are strange, being patches of darkness that exist surrounded by light, and which can at one instant perfectly mirror whatever casts them, and then in the next moment, become something unrecognizeable as the child of its parent. Thus, the worlds of Shadow are equally strange, typically existing as inverted almost-reflections of the true planet that casts them, but void where those planets are solid, vaporous where the reality is liquid, and constantly shifting between the various states of matter as time advances - sometimes in response to physical changes in the "world of light," at others because of new perceptions that have taken hold there, and in other cases for no more reason than that change is possible.
Just as life exists in the Light, so too can it exist in the Shadow, although whether it originates there or immigrates from the Light, it invariably absorbs the stuff and strangeness of its environment. Tricksters, shapeshifters, illusionists, creatures of fantasy and of nightmare - all these and more are native to Shadow. And since Shadow is so inherently malleable, moving from a world of Light into its respective Shadow - or indeed, moving between two worlds of Shadow - is often a feat more easily achieved than moving between worlds of Light.
That's how Briar ended up in the Twilight Realm once when she was younger. One of the natives, a humanoid being somewhat unimaginatively called a "Twili," played a little trick on her and led her through a portal into its home.
"It was the creepiest place I've ever been," the fairy says flatly. "Not as directly dangerous as floating around Sunnydale at night, but full of weird, well, 'shadows' of the creatures that I was familiar with, all dark and distorted - kind of like our unfriendly visitor out there. The Twili themselves are practically half-ghost, with all the intangibility and moaning, never mind that they really are alive."
"The use of shadow and mind-affecting magic I can see," you admit, thinking on Ganondorf's experience with the Twilight Realm and its denizens. Recalling Zant, you suppress a shudder. "But 'our unfriendly visitor' had a pretty big necromantic component to it, if the attacks it was using and the way it just ate my ki scans are any indication. That's not an aspect of Shadow, is it?"
Briar pauses in her scans, frowning. "Not really. Shadow's big things are illusion and transformation, with a side in enchantment and conjuration. Necromancy's all about death, souls, and undeath; if it were a significant part of Shadow, the Twilight Realm would be a lot more hostile to visitors than it is. For starters, eating the food would probably kill you, assuming that just being there and breathing the air didn't."
"So there's a chance that we didn't just have a run-in with a creature of Shadow," you say, "but rather a straight-up creature of Darkness?"
"That's a possibility," Briar admits. "You'd probably have to ask the wizard how the thing reacted to having lots of light thrown at it. True creatures of Shadow aren't really harmed by Light, even if it can be uncomfortable or inconvenient for them in any number of ways, but it burns through Darkness like nobody's business."
You turn to your English companion. "Any thoughts, Altria?"
"I would have to agree with your theory that this was a being of Darkness, Alex," the girl replies. "Ambrose has shown me how to at least recognize various fundamental aspects of magic, and this creature had a distinct aura of elemental Darkness. It also bore a certain resemblance to an entity I once read about called a 'nightwalker,' a sort of sentient mass of pure necromantic energy. If memory serves, nightwalkers are vulnerable to bright light and have the power to instill unnatural fear into those who meet their gaze." Altria frowns. "However, that same book also claimed that the smallest nightwalker on record was roughly twenty feet tall, rather alarmingly powerful, and indiscriminately homicidal. If this creature was indeed of that kind, and setting the question of its reduced size aside, no teenager should have been able to summon it, let alone control it well enough to stop it from killing her on the spot."
You look at Cordelia, who shakes her head. "I don't remember ever reading about 'nightwalkers' online."
"They are supposedly rather rare," Altria supplies. "And not prone to leaving witnesses that can testify to their existence."
Cordelia glances at Altria, and for a moment it looks like the brunette is about to say something to the blonde, something that might even be snarky.
Then there is a flash of light, a high-pitched yelp, and a fairy flying halfway across the office at high speed. Briar tumbles for a moment before getting control of herself.
"Oh, you did not just do that to me, buster!" she spits, brandishing her tiny wand more like a tiny sword.
"Er, Briar?" you ask. "What happened?"
The fairy pauses and takes in the three young faces regarding her. Then she sighs. "Okay. I have good news, I have bad news, and I have news that is just damn typical for the day we've all had so far. Which would you like to hear first?"