Elves are social creatures
Ferenblaud’s tragedy is that he isn’t
What we know about elven biology
The standard way to create a new elf is for an adult elf to ritually shatter their ruby into shards, each one creating a new elf whose ruby will grow over time (GM’s guide, pp. 52-54). Exceptions are if the shard was shattered incorrectly (e.g. Mard the Freak, Raven’s Purge pp. 210-211), or if the ruby is mounted in jewellery with no attempt at making a new body.
The Shardmaiden decided instead to make her heart shards an aid to Elvenspring druids rather than a source of new elves (Raven’s Purge, p. 21). If the ancient elves knew such rituals, what else could they have done?
Theory: elven rubies are more malleable than most people think
Legend says that the first elves were charged with “[filling] the world with life and beauty” (Raven’s Purge, p. 18), in particular organising and giving names to all the rivers and lakes (ibid., p. 19), so they could plant forests. Presumably similar attempts at categorising and promoting fauna followed.
What’s more likely? That a single elf wandered all over Ravenland, or that many elves split the work up between them?
My preferred theory is that Neyd split her ruby into multiple shards, forming many other elves who went off and followed all the rivers to their sources, then came back and shared what they’d learned, merging their rubies again.
Why can’t modern-day elves do this? Well, (1) maybe they can, (2) maybe you need a particularly-large ruby to start off with, because a ruby needs to be a particular minimum size to sustain a workable body (see Mard above), and/or (3) it’s possible that once two rubies diverge too much, because of different life experiences, they become separate personalities that can no longer be united again.
Theory: elves are social creatures who value differences
“Blood relation does not matter to the elves and they have no hierarchies beyond respect for age and knowledge” (GM’s Guide, p. 52), and I think a consequence of elves being immortal is that a lot of the squabbling, manoeuvring and ambition that you get in mortal kins just doesn’t happen for elves.
Humans, dwarves and halflings all have family structures that resemble 21st century Earth: who your parents are determines how much power you have in society, which can be built up, inherited and lost. While everyone is ultimately fighting against the Malthusian equilibrium, and a village will band together against a common foe (invaders, disease, monsters, natural disaster), they’re also competing against each other at the margins, for creature comforts, health and respect.
Elves are nothing like this, and I think that this lack of competition - which you can also call a lack of drive - explains their fundamentally egalitarian society. Given the tendency of an immortal to become tired with life because they’ve seen it all before, I think elves must be very interested in unusual things, in being surprised and challenged. And that especially means other elves.
The point of producing more elves, it follows, isn’t to have more warm bodies or anything so trivial, because elves don’t need that stuff. They’re not worried by age or health, and they’re nearly all of them ridiculously talented. No, the point of having another elf is to have a different elf. That’s the most interesting thing of all!
Reconciling the Bitter Reach and Raven’s Purge
The Bitter Reach says there was ultimately a fight between Ferenblaud and Blaudewedd. “Blaudewedd of the First, wisest among the elves, mightiest of all druids” (p. 45) “alone was equal to the Ferenblaud in power” (p. 88). The same page says the winter elves “are related to the elves of Ravenland but rejected the teachings of the Wanderer and Clay”.
Blaudewedd is said to be the founder / current leader of the Redrunners (e.g. p. 88), and the creator of the Stillmist (p. 45). The GM’s Guide (p. 54) says “The wise female elf Blaudewedd of the First is said to have stayed in the world to guide less experienced elves from her dwelling in the Stillmist, even though she is done living.” Meanwhile Raven’s Purge says that of the elves of the Heart of the Sky, Nebulos created the Stillmist (p. 19), and Gemelda created the Redrunners (p. 20); their contemporary Kalman Rodenfell is the primary commander of the Redrunners now (p. 46). There is no mention of Blaudewedd.
Whether the creator of the Stillmist was male or female doesn’t really matter for a kin that can reshape its body at will. More importantly, though: even if we posit that Blaudewedd was the “eldest sister, Gemelda, who was the wisest since the largest ruby dwelled in her chest” (p. 18), then surely she would have called upon her brothers, sisters and other allies in the fight against Ferenblaud? What would have been an even fight against just Gemelda would become a curb-stomp humiliation when facing all of the Heart of the Sky.
Proposal: Blaudewedd is the Heart of the Sky
If you decide that “Blaudewedd” means “any or all of the elves of the Heart of the Sky”, a lot of the difficulties go away. It’s true that Nebulos (in particular) and Blaudewedd (in general) created the Stillmist. Gemelda and Blaudewedd founded the Redrunners.
As for Kalman Rodenfell, that’s easy too: everything we know about the winter elves is that they think of having one all-powerful ruler, with even other powerful winter elves at best servants, if not slaves. So of course Ferenblaud’s opponent must be a similar elf of equal power. That’s the only thing that makes sense.
As to why we have this difference between the winter and summer elves, why Ferenblaud rejected the teachings of the Wanderer, perhaps the simplest explanation is this: when the large ruby from which Ferenblaud was born landed in the North, it didn’t shatter.
Blaudewedd is a group of elves, with Gemelda at best a first among equals, welcoming others like Algared and Kalman Rodenfell, living in peace and harmony. Ferenblaud, meanwhile, was born into a life of solitude from which they have never recovered, only encountering the summer elves many years after arriving in the Bitter Reach, at which point their personality was irrevocably set.
If you aren’t even able to conceive of a society of equals, then of course your reaction to discovering a large number of other kins is to subjugate them all. What other option did Ferenblaud even have?
And if the only other elves you’ve ever seen (the ones formed from smaller rubies, or from rubies who split on impact) have been significantly weaker than you, then when more, strange elves turn up, you’re going to try to subjugate them. That’s just how it works. And if they manage to defeat you, well, it sounds better to say there was one big elf. And one day - dreams Ferenblaud under the ice - they’ll pay.