Although there's a theory that Riven was the Noxian invader that killed Yasuo's elder, and that they have special interactions in-game like 'You cant run from your past riven, trust me, I've tried' or 'That wind technique.. Huh..'
I do not think that Riven is the elders killer for several reasons, I'm not going to go into full detail but in rivens lore it states that her longsword was given to her as a reward of being a higher-up in the Noxian military, and because she was able to wield such a giant sword that is twice her size, it states that the sword was infused with wind magic, and that is why she is able to use her ultimate 'windslash'.
She then later faked her death when singed turned on her, and her men during the Ionian invasion, as he was melting her men, she escaped and then, presumed dead, went to find a new life. She said she agrees with Noxus, but not the methods of how they're doing it, so it implies she is going to independently work on her own for the good of noxus, kind of like Talon.
When Yasuo's elder was killed, it was stated that he was killed with a wind technique, a technique that only one person has ever mastered in a whole generation, implying that wind techniques are hard and complicated skills to learn, master, and perform. Why would it be so easily accessible by a magic infused sword.
Additionally, Rivens sword was infused with wind magic, Yasuo's killer was killed with a wind technique, meaning that she couldn't of been the killer, as it was a technique that killed her, and Rivens sword is infused with wind magic.
That leaves the question about who actually killed Yasuo's elder? Is it an already made champion? Will it be a future champion in league? This theory really intrigues me in so many ways, there's also a bunch of other awesome league theories like 'Is zed nocturne?' or 'Is LeBlanc Jarvan IV?'
Man, lore really does interest me, anyways, that's my thoughts on that, peace!![slayer-jinx-catface]