If Rhaast is possessed version of Kayn, then who was Aatrox's wielder?

Assuming Aatrox is the sword, maybe the humanoid we see was the previous wielder. Yes, now changed to a demonic red hued figure.

We know that the darkin were imprisoned within weapons and that Aatrox is the only one who has escaped confinement.

"A Rioter involved with the story (...) only Aatrox among those imprisoned have escaped confinement.".

Which doesn't mean that Aatrox the darkin has escaped the weapon, but rather that he found a host to infest. Someone that got hold of the darkin blade.

Darkins are also very old, amongst the oldest beings, so they wouldn't have humanoid forms by nature. So "Aatrox being the only darkin that has escaped confinement" doesn't mean that the darkin is now free of the weapon, it might just mean the darkin weapon found a host. Just like Rhaast is imprisoned within the Scythe, up until the darkin manages to overpower Kayn, or someone else. If the darkin got free of the weapon they wouldn't look like humans, and maybe they can't ever revert whatever magic put them there, they only figured a way of going around the confinement, or rather, whoever put them in there forgot about that detail, or didn't think it would happen.

Also, we know that not anyone can "take" the weapon, it killed the Noxian who tried to wield it. So it would make sense that whoever took hold of Aatrox the weapon was someone powerful enought to do it. (So it begs the question, who was Aatrox wielder)

We know that the weapon can be by itself. Kayn uses the scythe for example, meanwhile Rhaast -- possessed Kayn -- is very similar to Aatrox (duuh). So the Aatrox that we know -- not the sword, the guy -- IS someone possessed by the darkin sword, the guy we see is the previous wielder... if there is anything left of him.

It might not be important to know, it might've been some random dude, or not, who knows (it's always interesting). But Aatrox as we know him has been around for a long time, so as every other darkin, but not every darkin has infested a host yet. Aatrox has.

Which brings me to the next point, does being possessed by the darkin weapon grant immortality?

Can the possession, miraculously, be reverted? And if so, would the wielder die or continue as he 'paused' when first possessed? http://leagueoflegends.wikia.com/wiki/Darkin

And now I look at Jax, someone who has been at the protectorate vs magelord war. Maybe that's why Jax uses a lamppost (the Fields of Justice is no more) --- His robes were heavy, but the sun’s warmth didn’t reach the mottled and oddly hued skin beneath. No part of his flesh was visible, which was probably just as well. He wasn’t even sure what his skin looked like anymore. --- Maybe he's someone that had a darkin weapon, and for some reason doesn't anymore. His robes might be hiding that, his defigured half-transformation.

The darkin weapon possession might not be a demonic horror movie type of stuff, but more in the lines of the "One Ring" where the promise of power is what gets you. There's a very similar scene in Kayn lore to that of the Lord of The Rings

https://giphy.com/gifs/zfctIcJgiDlHq/html5

"Brother, stop!” the acolyte cried, flicking blood from his katana. “What are you doing? You saw what it can do! It must be destroyed!" Kayn faced him. “No. It is mine."

Personally I like to believe that Jax and Aatrox stories date back to the protectorate and magelord war, maybe the magelord (Jax) had gotten a massive super weapon (the darkin) and they were winning because of it, Aatrox being the fella he is, decided to go toe to toe against this other darkin. Jax (supposing) either got the battle of his life and got his ass handed to him, seeing the bloodshed decided to walk away, or he himself ditched the weapon.

And so Aatrox's wielder might've been some mighty protectorate warrior that found the weapon at that time and quickly fused with it. We could even have him some day as a champion (I'm talking about the guy who took for himself the Aatrox blade).

If Kayn is worthy of a champion on his own... Would be cool if Aatrox wielder founded Noxus, and hence its cursed violent culture.

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Unrelated

I wonder if the could be the darkin blade Aatrox without a host

Coincidences of the blood theme aside -- look closely on the item icon () you can see that the Bloodthirster is serrated on the sides kinda like the Aatrox blade. Aatrox and the item Bloodthirster are years apart I think - and the latter could use an icon update (:

And being THE main lifesteal item that it is, kinda relates to the blood thirst Aatrox give man in battles.