UPDATE: More World of Warcraft: Legion secret-finding success! The epic mount The Reins of the Long-Forgotten Hippogryph has been found. This is the mount people were trying to find when they inadvertently worked out how to summon Kosumoth the Hungering.
And like Kosumoth, attaining the Hippogryph requires finding hard-to-find blobs and clicking them in the correct order. But this time the blobs are crystals, Ephemeral Crystals, and there are five of them. The first person to click on five of them within eight hours loots the prize, The Reins of the Long-Forgotten Hippogryph.
The Long-Forgotten Hippogryph is named such because it harks back to a hippogryph skin that was in the game data in 2007 but never actually in the game - hence only people in the data-mining WOW circles new about it. It's perhaps for precisely that reason that Blizzard brought the mount back and made it findable in the way it has, because only people data-mining WOW: Legion knew The Reins of the Long-Forgotten Hippogryph existed, and it was they who mounted a search to try and find it.
Bravo all round!
ORIGINAL STORY: Blizzard loves a secret (see: Sombra in Overwatch for a good example) and a whopper of one has been discovered in World of Warcraft: Legion.
That literal whopper is Kosumoth the Hungering, a giant sea monster world boss who kills everyone with one shadow spell if he's allowed to finish casting it.
Players knew Kosumoth existed because reference to him was found while data-mining Legion. That's how we know Kosumoth the Hungering drops an epic mount and epic pet (if you're lucky). People also came across him in Legion's beta because he needed testing.
But on the live World of Warcraft: Legion servers, Kosumoth isn't simply there - something needs to happen in order to spawn him. But no one knew what. Moreover, no one particularly cared. People were looking for something else.
The treasure on people's minds was a different epic mount, one that stretches all the way back to 2007 - to a hippogryph skin that existed in the game but was never seen or used. Hence the epic mount's name: Reins of the Long-Forgotten Hippogryph.
People knew from the data-mined description of the hippogryph that it is found in Azsuna, one of the four zones in Legion's new Broken Isles continent. But people didn't know where. So they looked; they scoured high and low for any clue - and they began finding things. Things that were incredibly well hidden.
One player found a sunken ship in the Broken Shores area - one of many such ships - and noticed a small crawl space in it. They went in, and they went deeper and deeper until they found another small hole - and this one led into a secret cave.
A comment underneath the reported cave discovery shows how obscure the find was: "Even with your pictures and instructions I could barely get to that cave," the admirer wrote. "It is absolutely insane you were able to find it."
Someone else found something in that cave: an orb - an orb just like the ones other people were turning up in the new zone. An orb just like the one next to orc hermit Drak'thul, who lives in the area, and who gives you a quest that enables you to click on the orbs and reveal them as Hungering Orbs. Hungering... As in, Kosumoth the Hungering. A connection was made.
Drak'Thul also provided another clue. On the table near his orb was an arrangement of pebbles, which players somehow decided to superimpose onto a world map of the zone and see what would happen. Well, what happened was they found the rough locations of all 10 Hungering Orbs.
After much searching the Hungering Orbs were found, and as they briefly said "attuning..." when interacted with, it was decided to try and activate them in some kind of sequence. The hunch proved correct. Following a lot of trial and error, a group of players assembled at each of the different Hungering Orb locations and began a sequence. A handful of activates had taken place when a message was broadcast on their screens:
"Azeroth itself recoils as unleashes an ancient evil.
"Kozumoth the Hungering walks the land once more..."
Kosumoth the Hungering had been spawned. The players who spawned him ran to fight him, only to find the enemy Alliance faction already there doing the same - apparently they'd heard the news too. Indeed, it appears that whenever anyone spawns Kosumoth, he spawns for the entire region of World of Warcraft servers, and everyone receives the message telling them he's there.
Kosumoth the Hungering was eventually brought down, then and many times after. He can be fought in small groups or large, and scales to suit the encounter. But the epic mount people are after, the Fathom Dweller jellyfish thing, and the pet, The Hungering Claw are extremely rare. Very few people have been lucky enough to loot it.
Nonetheless, more people have the Fathom Dweller mount than have the Reins of the Long-Forgotten Hippogryph. The search that started it all still goes on.
Yet, sometimes it's in the hunt that the real thrill is found. In the breathless words of the person who reported the successful Kosumoth the Hungering spawn: "This was most likely one of the most epic adventures I've been through in WOW. A big shout out to everyone who helped make this possible, it was a blast!"