Favorite headcanon for any of the Aen Elle characters?
I'll give a general one first (which relates to Auberon).
The idea that the Supreme Leader of the Alder Elves is the magical conduit tethered to the land of their realm, and that their capability & well-being is linked with the flourishing of the land.
This is simply a symbolism of sacred kingship made literal. We know, elven magic is different & elves have a special connection to the land that they live on; they are integrated by the territory, as we learn in BoF. Adaptation for elves, it seems, could imply adaptation to or OF the land they call their own.
By way of an example, Auberon is a wounded ruler of an idyllic-seeming, timeless realm that upon reflection seems a lot like a magical pocket dimension within its own borders. There are signs of the withering and trouble in paradise (or paradise's approximation), but overall, the fundamental terrestrial magic of Tir na Lia still holds. Otherworlds are frequently lands of eternal abundance and bliss, fertile in many a sense: not only the land itself "gives" freely but most mortals' adventures in otherworlds also include sexual dalliances. The place breeds life, but in essence remains still something of a shadow world.
As Ciri discovers, the realm of the Aen Elle only looks beautiful; there are unknown dangers beyond the land that they have cultivated into their eternal garden & protected with time-space warping barriers. So I like to think that any lands that the elves settle they bind in various ways to their essence, their forms of magic which allow them to manipulate its physics, its landscape, its biosphere & life, its climate.
Take, for example, the question of elven agriculture. On the one hand, Filavandrel suggests elves never cultivated the land by ploughing. On the other, in order to sustain a civilization hunter-gathering does not scale (with a standing army, no less (IFF that was, in fact, the case pre-Conjunction)). The earth, Filavandrel puts poetically, gave birth and blossomed for the elves because it loved them. I'd wager love had nothing to do with it, but an ability to both seek symbiotic relations with the land and/or - when the former failed - transforming its geography-specific peculiarities might have.
[A friend (SMiki) recommended a few symbiotic modes of agriculture recently, which make for a nice reading too.]
Furthermore, humanity's entrance into the world, according to Filavandrel, changed something fundamental about the world itself until the land began responding better to the human way of doing things than to the elven way. So either this is metaphysical bullshit attesting to the bitterness of those who lost power (a very human view, that) or there was & is something structurally different in the way elven civilization through its magical abilities is able to influence the material world. Maybe a bit of both, and maybe elves are a bit like a cordyceps-like species in their own right; except with more delicate tools (for the non-magical human eye).
Aen Elle are a good exhibit. They not only left for a world in which they could become the dominant species, but they also deem their covertly dangerous world "interesting." Which, unless they are fond of the Weird (which they very well might be), could entail some form of taming. I think elves are able to make the land on which they found their realms respond to them as they would like with various degrees of success (their fertility rituals - like the ruler's marriage to the land, for example - I hold as more "literal" in their effect upon the world, whereas for humanity rituals are merely symbolic), but to secure this success and maintain it in a stable state requires capital Magic & self-effacement. Which is why Sages are the elite that they are among other things, and preferred as High Kings. It does not exactly guarantee a trouble-free existence, because if the "body" of the conduit suffers some manner of abrasion, infirmity or madness, so does the architecture of the Aen Elle's pocket dimension, but its thematic and vibes fit with the elven lore; in Sapkowski & otherwise.
I've opened the topic in Games.
(Where I also think that just as magic can alter the nature of reality toward the fantastic, cool human rationality (as elves put it) & scientism can alter it in the other way too. And the Conjunction caused the two different pools of realities to clash & become temporarily intermingled. Causing trouble.)
Going to try and pick individual character HCs later, hopefully. Thanks anon!