Originally Posted by
Soulwind
The Broken Isles WERE under the ocean.
That said, I don't think it's the prison of N'zoth. I don't know what was the purpose of this facility, and the room were Aegwynn put the Avatar of Sargeras is EXTREMELY convenient, fel containers and all.
I doubt Blizzard gave it much thought, but considering how deeply buried and unconnected it is to the surface, it could be an insignificant world-shaping machine, something they used to monitor or manipulate the ocean or the magma below it. It just happened to be in the right place when the night elves built the temple, which later became the tomb.
I'm just disappointed the "labyrinth of massive corridors covered in arcane runes that turn the building into a complex magic key, filled with traps and ever-shifting" described in the audio-drama is just an indoors Nighthold.