So after taking a closer look to the images on the Noxian dairy I noticed something peculiar in one of them
Etched to the mountain is a mural of some sort in with on top we can see Rek'sai
But in the bottom part look like 
both locked in combat!
How much does Rek'sai has lived in Shurima!?
I like to think Rek'Sai is actually older than Shurima, as in when it was a city. Back when it was just desert as far as the eye can see which would make sense since Renekton would still have to be sane for this picture to be accurate. Since he was the defender of Shurima before the whole fall of Shurima thing happened.
EDIT: WAIT could this? Could that picture show Renekton but....before he was Ascended? His face is covered up, maybe implying that he wanted to forget who he was and only be known by what he is. Or perhaps it could be a Shuriman who defiled his face because Renekton has betrayed them all? So many theories!
Cool find! Maybe the threat 
and ascended to combat was the arrival of 
and other void creatures.
shurima is one of the oldest civilizations in runeterra after freljord... that means that rek'sai could potentially be older than vel'koz, who was deffinetly accosiated with ancient freljord... interesting
It's hard to make out, but I can clearly see the handle of Renekton's blade. Nice find.
Rek'sai was probably defending her territory and or her kin because as the Queen of the xer'sai that's her duty right?
Renekton was once the Ascended being who defended Shurima from Rek'Sai. Then he went mad at Rek'Sai began to devour Shurima in the wake of Xerath's betrayl.
Whoa good catch there it really looks like that is an ancient picture of the two battling.
Since both are pretty old it is possible that they clashed once before the fall of shurima.
Since ascension is seemingly granted,I'd say it is not unlikely that the ascension of the 2 brothers might be related to Rek'sai.
EDIT: Just noticed that Renekton seems to be human in that picture. Notice how small he is compared to Rek'sai and his arm and leg don't seem to have any scales.His head is kinda hidden too.
I couldn't make it out the first time but now I totally see Rene's weapon.
