Details surrounding the Icathian Cataclysm

** Nasus and Setaka were part of the army that fought in the battle **

• In Bloodline part IX, Nasus tells Sivir that Setaka had been brought down by monsters at the Icathian cataclysm.
• It is not particularly unlikely that Setaka was the leader of God-Warriors in Where Icathia Once Stood as she is also mentioned as "Warrior Queen of the Ascended Host" by Nasus

It is the Chalicar, the blade once borne by Setaka, foremost Warrior Queen of the Ascended Host; [...] but she was brought down by monsters on the eve of Icathia’s doom. I held her as her light dimmed and I sent her to her slumber far below the sands with her weapon upon her breast.”

** Ryze's master, Tyrus, played a significant role in the events surrounding the cataclysm. **

• In Where Icathia Once Stood the new weapon is presumably a world rune due to the immense magical power it holds.

I did not know what they were doing, but I heard an insistent buzzing sound, like a hive of insects trying to push their way into my skull.

The pavilion’s outline rippled like a desert mirage, and I had to look away as my eyes began to water. My teeth felt loose in their gums, and my mouth filled with the taste of soured milk. I gagged and wiped the back of my hand across my lips, surprised and not a little alarmed to see a smear of blood there.

“What is that?” I asked. “What’s in there?”

Saijax shrugged. “A new weapon, I heard. Something the thaumaturges found deep underground, after the earthquake at Saabera.”

• In Ryze's Bio it is explained how Ryze heard of what happened in Icathia and rushed there only to find his master responsible for the catastrophe
• It would seem that Tyrus used the world runes to stop Icathia from abusing the world rune(s) they had found any further.

Ryze was on one such menial errand when he received word of a new cataclysm, this time to the southwest of Valoran, in Icathia. The mage rushed to the scene of the devastation, beset with concern for his master and friend, hoping that he had survived. Upon his arrival, Ryze was elated to find that Tyrus was indeed safe. But Ryze’s relief was short-lived. Next to the scroll he had never been allowed to read, sat two World Runes. The older mage explained that once the World Runes were in play, he had no choice but to use them himself.

Just because I can't leave it out: Saijax is Jax

• We know he's from Icathia and loves eggs from his color story and bio

EDIT: It seems there is some dissonance in the timeline concerning the Ryze theory

as noted Tialilia

Ryze is alive in the time of the RUNE WARS, which happen after the Fall of Shurima, the Darkin War and Mordekaiser Reign. The Void War/ Fall of Icathia happens in time of the Shurima Reign.