In Neeko's lore, Neeko is the last of her tribe of the Oovi-Kat, but it never said what killed off her tribe.
"Cataclysm loomed on the horizon."
and from Neeko's VO
"Neeko doesn't want to be here when the OTHERS arrive."
Two things most likely happened to them, a void rift opened near them and a creature of the void attacked, or human settlers found the island and killed off Neeko's tribe.
Harrowing. Since the Shadow Isles were so close to her home Island.
"Neeko rested where she could, calling out into the spirit realm for her lost tribe. She felt only scattered, sad echoes in response, and images of towering, dead trees that lay somewhere over a fragile horizon…"
It's a big enough hint right there in the biography.
I don't think it was the Void or humans. Granted I don't think she ever saw the attackers, but she doesn't seem to fear humans or the Void(this based on her in-game quotes, where she asks if she can touch Vel'Ko's eye, among other things).
Thanks to the quick thinking and self-sacrifice of the Oovi-Kat elders, Neeko escaped the death of her homeland. She clumsily took the form of a bird, and fled the smoldering destruction, feeling the screams of her people fading into the ethereal gulf between realms.
With the last of her strength, she crept aboard what turned out to be a mercantile vessel destined for Harelport. Neeko rested where she could, calling out into the spirit realm for her lost tribe. She felt only scattered, sad echoes in response, and images of towering, dead trees that lay somewhere over a fragile horizon…
But the towns and cities, with all their flaws, still called to Neeko. Her ancestors came to her in dreams, showing her the pale branches of those dead trees, over and over. The trees needed color, to bloom again—of that much, Neeko was certain. She asked her friend to join her on this new journey, but Nidalee could not be persuaded.
Nothing about that sounds like the Void. Humans are a slight possibility, though.
We know humans mated with the Oovi-Kat at some point, as they have human blood in them, but not as much as the other Vastaya due to their isolation. Its not entirely out of the question that humans found them again.
Though, if I'm being honest, it sounds something like a volcanic eruption. Whatever happened happened suddenly, so quickly that only Neeko could escape. A conflict with humans, or even the Void, would have lasted longer than that.
Considering that it was a thing that destroyed a whole Island, I think it was Aurelion Sol... Not that he wanted to kill them, but as a "casualty" of the attack that he used on "Twin Dawns"
