Originally Posted by
DeicideUH
Has always been there. Even when Hyjal was off-limits in Vanilla. It was Warcraft 3 lore.
But it is not the original Well. The original exploded, and Illidan kept six (seve?) vials of its water. Some (three?) of the vials were used to create this second Well of Eternity in Hyjal, and that's the reason Illidan was called traitor and imprisoned, as the night elves had forbidden arcane magic. The world tree Nordrassil was planted over the Well to hide it/contain its power. In the Third War, Archimonde tried to absorb the Well's power, as was shown in Warcraft 3 and retelled in the Caverns of Time Hyjal raid in TBC.
One of Illidan's vials was also stolen thousands of years ago by the highborne and used to create the Sunwell.
I don't remember what happened to the remaining vials.