More likely is, depending on how well he’s come to terms with his disowning, that it will be a topic brought up much later if they have rough times. I do not believe him foolish to get disowned, then abandon her whom he was disowned for.

As I recall, Vae’s home, while more underneath than not, is actually inside other things(a galangal root, a cleft in the world-amber)… but then, Vae herself is unusually good with Locador, even for a nendrai; she has a quarter-inch high treasure room that can comfortably contain a ten-foot-high armoire, and a demonstrated ability to redefine the local meanings of “up” and “down”. She is probably one of the more normal-looking things that can live comfortably in the Underneaths, and she can do so at least in part because she makes them not behave as though they ARE underneath.

The less mighty-in-magic things, relatively speaking, that naturally live there presumably look more bizarre – Vae’s odd self-transformations notwithstanding. Though I am envisioning collective creatures – not so mighty as a chromodon(although there may be some such mighty things that favour the Underneaths), but, say, something akin to a swarm of spiders or other surface-clinging creatures, with a single mind.

Some inspiration might be had by looking at cave creatures – the lack of light, the harsh surroundings and difficulty finding food – except that there is, of course, NO ground worth the name, nor running water.

Some further inspiration might be had from the Lovecraft or Giger mythos… witness the ororosti(which, granted, I haven’t seen much detail about; can’t yet afford Marriage, sigh).

The Underneaths are extremely wild and dangerous, and their ecology is strange and upsetting. I don't think there are any Underneaths-beasts in the rulebook, though nendrai can live there comfortably if they like -- Vae does, IIRC, back in Vheshrame. A Marriage of Insects shows off some other Verticals-beasts: ororosti like strings of eyeballs, floating tetrahedral vorwi, and mighty-winged nycathath as you have seen in Sythyry a little.]

Given the fast distances between branches, anything that falls that far without some means to slow its descent will be very, very dead. And the physics of air are strange enough on the World Tree that simply being small might not be sufficient.

In any case, things that live on the sides or undersides of the world-branches are well adapted to life there. Sleeth may live on the Verticals(the sides), but even they are likely to move into, or at least closer to, cities when ice makes the footing treacherous; more commonly, things like jacks o’ hooks(which have great numbers of cable-like tentacles, very strong, good at gripping the sideways trees) or things that can outright fly are more likely denizens. As you get underneath, this becomes even more true.

At any rate, I’m pretty sure even Sleeth don’t live on the Underneaths. Too many very dangerous beasts do, with not enough places to run and jump.

Re-read the first chapter in detail for a little more information; for details on some specific beasts that live in the Verticals, see helletumug, jack-o’-hooks, and jaran-jabow on pp 271-273, in the Bestiary. Jacks at least are tough enough, and good enough at handling vertical spaces, that they might be found in the Underneaths; they are also very alien to our concepts, which can pretty much be expected of anything that lives there.

Walking directions are worse – walkward and outward twist and bend as the branch does, and rollward and roll’gainst(the walking directions, not the similarly named navigational ones) are perpendicular to them. They are not easily reconciled between two places – even if a spur from one branch comes closer to a spur of another than their respective main branches are to either at that point.

Thus, the navigational directions, which are always relative to the main trunk of the Tree; you can, thus, keep heading inward or outward and not have it change on you, but rollward and roll’gainst still cannot be strictly followed in straight lines – they are circular. There is a simple Kennoc spell(Find the World, I believe it’s called) to tell you which direction is the main trunk – but, of course, Kennoc spells are not entirely reliable. Those who live nearer the main trunk than any others have it easier; they can navigate by sight.

In contrast, and barring odd Locador magic, “up”(or, on the same axis, “down”) is the same direction wherever you are, and won’t change no matter how far you go in ANY direction. The “root” of the Tree, if it has one, is down; Ketheria is the uppermost ring of branches, or at least was(more seem to be growing above it); the upper side of each world-branch is dished.

(By the way, Bard, I got a copy of the sourcebook and have been slowly reading it as time permits. Makes a bit more sense – the Tree is much much larger than I had thought initially, and the innate unfairness-to-monsters of what it means that there are Primes is offset by learning that there are enormous areas where there are NOT, and even more, realizing that there are most likely a LOT of other civilizations over the thing. And … in one place, it is revealed that up is up and down is down, but what does that mean?
Up is crownwards, down is rootwards, or up is away-from-tree?)

I’m not sure *why*, but I get that vibe.