I’d so do this quest, on every character I own.
On topic, it’s a little bit more than 10000 years ago now 
It was a massive point in Azeroth’s history, as others mentioned, the War of the Ancients.
The Legions invasion of Azeroth, the Highborne and Azshara’s use of the Well of Eternity drawing Sargeras to this world. The battles against the Legion, the destruction of the Well and the Sundering, splitting the world apart, Illidans imprisonment, Elisande sealing Suramar City from the outside world, so much happened back then 
That kind of massive apocalyptic event tends to leave a mark, not just on the planet (The Maelstrom, the shape of the continents etc) but in the history of the world, it’s peoples and their stories.
There’s other big events throughout Azeroth’s history, but that one really takes the cake.