So from the Shadowlands features trailer:
The part that begins at 0:30 is what I am referring to. Bolvar narrates:
"Within the realm of shadow, lies the darkest of terrors, which should never be set free."
And this is the scene:
So that is most definitely Mueh'zala, right?
Or this "Jailer" perhaps?
Or if not, would could possibly be considered the "darkest of terrors" that should never be freed? Certainly Bolvar knows what he is talking about being the Lich King for the past 4 - 5 years. We've encountered a lot of terrible things. Sargeras, N'Zoth... but the darkest of them all?