The place
The Barrow Deeps are an ancient cave system beneath Mount Hyjal. They had two uses, and they sat uncomfortably next to each other: they were where the Druids of the Claw slept through their long hibernations, and they held prison vaults.
A dormitory and a dungeon in the same rock. The night elves did not see a contradiction there - both were about keeping something still for a very long time.
The prisoner
Illidan Stormrage.
After the War of the Ancients he created a second Well of Eternity - the thing the war had just been fought to destroy, made again, by one of the people who had fought in it. For that he was imprisoned in the Barrow Deeps.
The sentence was ten thousand years.
The jailers
An order was built around one cell. The Watchers, under Maiev Shadowsong, existed to guard him, and they did it for a hundred centuries.
This is the detail worth sitting with: Maiev's entire character - the obsession, the hunt, the refusal to let go - is the residue of a job nobody ever relieved her from. She did not become a warden. She was made into one and then left there.
How it ended
Tyrande Whisperwind freed him. Not out of mercy: she believed he would be a powerful ally against the Burning Legion, and she needed one.
Ten thousand years of containment ended as a tactical decision made under pressure. Everything Illidan does afterwards - and everything Maiev does after that - runs from that moment.
Careful with the name
There are two similar names, and the wiki flags the confusion itself.
The Barrow Deeps under Hyjal are not the Barrow Dens in Winterspring, which are a separate location. There are also Hyjal Barrow Dens, and whether those are connected to the original Barrow Deeps, or replaced them, is not settled. Anyone searching for "Barrow Deeps lore" will land on at least two different places.
What Forever does with it
The Barrow Deeps become a 10-player raid at max level, unlocking on 9 December 2026. Blizzard describes it as riddled with corruption, with new tier sets and a legendary reward still under wraps.
What the raid is not, so far, is explained. Blizzard has not said whether Illidan's cell is in it, whether the Druids of the Claw are still down there, where the corruption comes from, or whether the Watchers appear at all.
That silence is worth noticing rather than filling. The place already carries one of the heaviest stories in Warcraft; a raid set there is either going to use it or step carefully around it, and nothing announced so far tells you which.
The raid's confirmed details - size, unlock date, what is known about the encounters - are on the Barrow Deeps page.