Due to the nature of this story, it was inevitable that certain questions or confusions were going to pop up more than once. Since worldbuilding is a thing that I quite enjoy, I have a quite-large text document detailing certain elements of the world, which has expanded over time to include explanations for questions that have cropped up frequently in the comments.

Certain sections will be redacted out due to spoilers for upcoming chapters.

Twilight's Origin Universe

During plot layout, Twilight's home universe was essentially described as 'FiM, but if none of the plot ever happened'. So, no Nightmare Moon (Luna and Celestia never had their fight), no Discord, no Sombra, Cadance is a normal pegasus from some backwater town (probably hollowshades), Twilight's 'incident' during her entrance exam never occured (she was still admitted, just not as Celestia's student. By extension, Spike doesn't exist), and so on.

The Ruins

The ruins are a physical 'crack' in the multiverse, caused by the expentidure of obscenely-high-energy magic, [REDACTED RE:CH26]. Historically, creatures capable of generating and utilising similarly extreme magical energy levels tended to occur in roughly the same physical location in the multiverse, and over time, these multiple cataclysmic Detachment Events created a large fissure in the large-scale structure of the multiverse.

The name 'Ruins' refers both to the fissure itself and to the worlds that reside within and immediately around it. Outsider Beacons and Interworld Teleport spells cannot function within the Ruins.

The Falls

'Falls' is a descriptor used for worlds that lie within a specific physical band of the multiverse, specifically those who are near the Ruins, or in danger of suffering a Detachment Event. In practice, the term is used to refer to any actively hostile world that could pose an existential threat to the Exterior or Outsiders (either due to local wildlife, hostile inhabitants, or high-level magic users).

The term is entirely literal, since worlds within the Falls are more prone to 'falling' into the Void.

What are the two-letter designations from Lunatic's teleport spell in Chapter 6?

Since Fall worlds tend to pop up more often in Outsider mission planning, they are given a simple two or three-letter designation for common use so Outsiders don't have to use the full navigational identifier. When originally written, some of the designations were intended as easter eggs (referring to other fic universes), others were arbitrary generic designations, and some were just random sets of letters with no real significance. The following is the list from Ch6, as well as their corresponding universes. Some (such as NV, UP, and RW) tend to have many individual worlds that fit under their descriptor, so they are followed by a single numeric identifer (eg. NV8, RW5)

RF - Arbitrary (Random)

FO - Fallout: Equestria

HR - Hard Reset (The Outsiders don't know it, but high-end time manipulation magic is very detachment-inducing, though it tends to cause very clean detachments)

NV - Nightmare Victory (Any world where Nightmare Moon succeeded against the forces of Harmony)

CI - Celestial Intelligence (High-tech worlds with hostile AI)

UP - Underclass Poison (Worlds with notable tyrannic caste systems)

OB - Arbitrary (Random)

RW - The Roaming Wastes (Environmental Calamity, Partially-Radioactive Wasteland)

EF - Eternals Forgotten (Anti-Alicorn world)

What was the creature in Chapter 6?

An Elk. Since that world tends to have a large turnover of hostile elements, a number of non-pony races have risen to give challenge for the post of 'Ruler of Equestria', some a bit more exotic than others. I briefly toyed with the idea of the current tyrant of that world being a giraffe, but I felt it would be a bit too silly and went with Twilight, instead.

It is not, as feared by some readers in the comments section, an FoE-inspired Caribou, though after exploring the background material, I can see why some readers would draw paralells between them (apart from the obvious misogynistic tyranny thing). The Elk's larger stature is mostly due to physical training (I pictured it as a bit mongol-esque, given Elk's typical areas of habitation), and the anti-magic armor is a technology native to that world, developed as a defensive measure when attempting to combat and capture Alicorns (see below for further details).

I'll admit, though, I find the essential idea of Viking Caribou to be interesting (and seriously, 'Viking Caribou'. It sounds cool), though i'm dissapointed that its most notorious execution is as part of an excuse-plot gorean fetish fantasy. Someone needs to take the Viking Caribou concept and actually work with it properly (though given my tendency to go 'Fuck it, I'll do it myself' in the past, I may indeed just do it myself).

Anti-Magic Armor?

For context, when writing and/or designing fics or stories for FiM, I tend to use the RPG system 'GURPS' as a sort of pseudo-headcanon, GURPS Magic specifically (with a few house-rules tacked on to help with balance and lend usefulness to spells I felt were a bit neglected, like Ward). The included spells (of which there are about eight hundred. Not even kidding) encompass practically every magic showing seen on the show thus far, so I have a fair bit of leeway in what I can throw together. Also, since it's ready-made, I don't have to come up with a suitable magic system myself (and, inevitably, if I tried to make something myself it would end up something like the magic from Sam Hughes' 'Ra', very mechanical and rigid).

The Elk's armor is inlaid with a decorative jewel. The jewel itself has the enchantment 'Magic Resistance' embedded into it, alongside a 20-Thaum 'Powerstone' enchantment (which provides the casting energy for the spell). When activated, the enchatment provides +20 resistance vs. magic to the wearer for 120 seconds before the powerstone charge is entirely depleted. The always-on 'Amulet' enchantment isn't used in this case since the magic resistance works both ways, mitigating not only inbound magical effects, but also impeding the casting of any spells.

It's worth noting that not only is the armor itself not subject to the resistance (So you could cast, say, 'Disintegrate' on the armor and it would work as normal), the resistance only works against direct magical effects. Attempting to cast 'Ignite Fire' on a Magic Resistant object (or person) would fail, but hitting them with a fire-enchanted sword would work fine (throwing a fireball would also work).

Would FoE exist within The Interior?

Just going to nip this one in the bud on the off chance that it pops up again (since quite a few people seemed to jump on the 'IT'S FoE! DWAMA!' bandwagon when Chapter 6 aired)

FoE does not exist within The Interior, for a very simple reason - Anthro worlds as a group do not exist within The Interior. Humans and humanoid creatures as a rule do not exist on Interior worlds (The crystal mirror to the human world DOES exist on many worlds, but it exists at right angles to this fic-verse and doesn't really ever come into play), with the exception of the obviously-fantastic (stuff like minotaurs, centaurs, and the like). This is primarily for simple stylistic reasons, but it also stems from the fact that I personally don't quite GET why people make anthro versions of FiM. Giving them humanoid stature but also retaining equinoid features like wings, tails, and horns smacks of trying to have your cake and eat it too.

Of course, given that the underlying structure of the Interior allows for quite a few possible permutations and variations, a non-anthro FoE equivelant could potentially exist somewhere in there. However, as I've mentioned before, FoE isn't something I intend to explore in my writing. Ever. EVER.

What is the structure of the Library?

The Library Index is a single spherical chamber, 1536 kilometres in diameter. The interior wall of the Index is divided into 92 sub-indices of varying area (Specifically, the pattern is that of a Snub Dodecahedron), each containing the complete Index for a given subchamber. These index shelves extend 12 kilometres downwards from the surface, and contain dedicated portal doorways to the area in the corresponding subchamber for which that subindex describes.

There are 92 subchambers. Subchambers linked to from the larger pentagonal subindices are structurally identical to the Index Chamber (1536km diameter, 92 delineated subsections for ease of navigation), with bookshelves extending to a depth of 192 kilometres. Subchambers linked from triangular subindices are smaller (768km diameter, 92 delineated subsections, 96 kilometre bookshelf depth)

The Index has a maximum book-storage volume somewhere in the range of 300 million cubic kilometres, and can contain at most 1.25e+20 volumes of index information.

Large Subchambers have a bookshelf volume of 6.4 billion cubic kilometres, and can contain at most 2.7e+21 volumes (There are 12 subchambers of this size, for a total of 3.2e+22 books)

Small Subchambers have a bookshelf volume of 800 million cubic kilometres, and can contain at most 8.3e+19 volumes (There are 80 subchambers of this size, for a total of 6.66e+21 books)

How big is the Exterior?

It has a functional volume several hundred billion times the size of the observable universe. The amount of that space that is actually utilised by the Outsiders is significantly smaller.

How big is the Interior?

Outsider short-form teleport addressing schemes allow for 68,719,476,736 (16^9) potential target worlds. Long-form addressing allows for (holy crap digits!) 4,722,366,482,869,645,213,696 (16^18) target worlds. Short-form addressing is considered sufficient for general operational use, with long-form used for greater precision when targetting worlds that are abnormally closely packed. However, much like how the size of the internet isn't equal to the number of possible IPv4 addresses, these numbers serve more as upper bounds for the number of worlds that are accessible.

In truth, I never really wrote down a number for how many worlds exist.

Structural Layout of the Exterior

The Exterior doesn't occupy a physical space as is conventionally understood. The best way to understand navigation in the Exterior is to do away with conventional grid-based visualisations, since the Exterior is actually configured as a set of interconnected nodes. Each node (functionally, a node is a single room) has one or more interconnecting portals (doors or openings), which are linked to another portal in a different node. This underlying characteristic gives rise to a number of unusual structural features, such as a room that is seemingly 'bigger on the inside', or a door that can lead back to the same room in a seemingly infinite loop.