Why do I get the feeling that Riot has completly forgotten the Darkin?

Because since Aatrox's rather underwhelming release we haven't heard anything of them. And that was quite some time ago... Wasn't 2015 supposed to be the year of "mythical and legendary champs"? Well, we got a lot of them, even freaking death himself, but still no news on the Darkin.

And the two darkin candidates we have? Jax is essentially lore-less. He might as well not exist at this point as his lore is 100% centred around the IoW. And the other candidates...

Well, but what do we even know about Darkin? All we know that Aatrox, "the darkin blade" probably refers to a sentinent sword and the body is some kind of blood-powered golem. Or some posessed dude who doesn't seem to age, who knows? So, if Darkins are non-human-things that take on a humanoid form. But what are these "non-human-things"? I'd go as far and say that they are "concepts" (or more specific, "Jungian Archetypes") of certain things - it wouldn't be wrong to call Aatrox the material manifestation of "War" (or rather "Bloodlusty carnage"). And what is his form? A blood-drinking sword. Seems to be pretty fitting here if you had to find one thing that shows "war" or "slaughter".

Going from the fact that Aatrox is war and that Riot Designers are PROBABLY influenced by western history and culture, they might have been going for that "four horsemen of the apocalypse" theme a little. That 4-man group is a bit varying in it's members depending on from what cultural context you take them, but usually two are always present: "War" and "Hunger".

While there might be as well Darkin that might represent other themes apart from the 4-spirits-on-horses group, but I doubt that there is not a "hungry" Darkin already in league.

Let's thing a bit, what is a symbol that really is affiliated with "Hunger" in our culture and history? You can't really pin it down on an object like with war/slaughter. So maybe it's not an object, but an animal? Pigs are usually seen as symbol for gluttony, which is not hunger. Goats eat pretty much everything, but are not really brought into context with it either. But there is one: it is: the black bird. Crows or Ravens in that case

Usually this would apply to any bird that would eat the meat of corpses, but in medival/post-medival europe (which seems to resemble runeterra in some ways) crows/ravens were the most common.

Wait, we don't really have a black bird-champion in league, do we...? Oh wait. He might just get a new splash art...

But how is Swain hungry? "MY BIRD IS HUNGRY!" "LET'S JUST MORPH AND EAT THEM!"

Ok, wrong question, how does his actions and his rise to power fit along with the "hungry" theme?

Well, you can be hungry for other things than food. Honor, Stenght, Political Power... and since in Noxus strengh (in all of its forms) = Political Power & Honor, one shouldn't be surprised to find the literal manifestation of hunger somewhere in there.

I have seen one interpretation of Swain of him being a magician that made a pact with some kind of crow demon (in that case the Darkin), but that's not really it. The human body might as well be a "fake", the real Swain being the bird. Might explain why he needs a crane to walk. Birds can't normally use that walking pattern, they usually usually "hop" more than they "walk". He's not injuried on one leg. His mind is just too "birdy" to let him walk without one.

That doesn't exclude the fact that "Beatrix" (the bird on his shoulder) is not an existence of her own. They could have some kind of mental/existencial link and still be two seperate beings. One forming the more intelectual aspect, the other the more natural of the Darkin of Hunger.

Tl, dr: Swain is Aatrox smarter big bro.

Ah btw, I'm not forgetting Varus here. When the theory of Darkin "posessing" people is true, then the manifestation of "Corruption" might be quite thing that Varus has sold his life to. I mean, look at his kit. Corruption is usually seen as something "weird, unnatural and harmful that expands" - let's compare that to the ult... or the fact that he defiles the very ground he shoots his arrows on...

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I wonder if we will ever get any updates on this... Well, maybe Swains gets some expanded lore... he would deserve it.