The Warbringer

This Nazjatar vision contains a key scene from the Sundering which transforms Queen Azshara and her followers into naga and triggers the Cataclysm.

Wowpedia describes the Sundering:

Once the beloved ruler of the Kaldorei Empire over ten thousand years ago, [Queen Azshara] was entrancingly beautiful and beloved by her people. When her Highborne made contact with the Dark Titan Sargeras and his Burning Legion, she accepted his offer to cleanse her world of the impure, of the inferior races. The Highborne and the demons worked together to summon the full might of the Legion and their master on Azeroth through the Well of Eternity, but the Kaldorei Resistance thwarted their efforts.

The Well of Eternity was destabilized, and collapsed, swallowing immense swathes of land with it in an event known as the Great Sundering. Azshara and many of her Highborne followers fell into the depths of the newly created Great Sea, and would have drowned if not for the intervention of the Old Gods. They forged a pact and saved the Highborne by transforming them into the serpentine naga, a price they would pay by becoming their servants.

https://wow.gamepedia.com/Queen_Azshara

World of Warcraft Wiki contains this description:

The Great Sundering, also known as simply the "Sundering" or the "Cataclysm" (not to be confused with the cataclysm caused by Deathwing), was a world event which reshaped Azeroth approximately 10,000 years ago at the end of the War of the Ancients. This was caused when Queen Azshara created a portal for Sargeras in the Well of Eternity. The spell created an unstable vortex within the Well's turbulent depths. When those channeling the portal were attacked at the Eternal Palace, it threw the carefully crafted spellwork into chaos.

The unstable vortex within the Well's depths exploded and ignited a catastrophic chain of events that would sunder the world forever. The massive explosion rocked the temple to its foundations and sent massive quakes ripping through the tortured earth, as the surging Well of Eternity buckled in upon itself and collapsed.

https://worldofwarcraft.fandom.com/et/wiki/Great_Sundering

Also see the Wowhead guide:

Lore of Queen Azshara, Ruler of the Naga

https://www.wowhead.com/guides/lore-of-queen-azshara

You can view the cutscene again by interacting with the npc [npc=155089]. Coordinates 79.8, 45.0. See map above.

Really long cut scene, go to the bathroom first.

The Highborn City has a lot of resemblance with Santorini, kind of ironic to see how that glory goes down just as Queen Azshara herself.