Somewhere in the Endless Sea, an unnamed location on the map.
Although the main forces of both the Alliance and the Horde were currently tied down in the Broken Isles region of the South Seas, they were still long-established military alliances. At the Titans' call, both sides scraped together oceangoing fleets and gathered one after another in this barren, nameless stretch of sea.
Oh, the cause of it all was nothing much. After Old God Miss Salathas finally got herself a physical body to walk the mortal world in, she was immensely gratified. Then, after Boss Hu sweet-talked, flattered, threatened, and tempted her, she ultimately climbed into Boss Hu's fragrant bed... ahem, ultimately agreed to Boss Hu's request.
After that, drawing upon her years of experience working for the Void Lords international corporation, as well as her years of experience alongside her fellow Old Gods, Salathas combined that with geographical knowledge from after the Great Sundering of the World of Azeroth—ocean currents, tectonic shifts, and whatnot. Anyway, it was hard to explain, but in short, Salathas successfully located N'Zoth's Seal Cage, making Boss Hu leap up from the bed in delight... ahem, leap out of the room.
What happened next was naturally no surprise. Boss Hu was a straightforward man who did not beat around the bush. After returning from the divine kingdom planet's shipyard, he did not even bother going to the Broken Isles. He immediately assembled his forces and charged boldly into the depths of the Endless Sea.
"If my calculations are correct, the seabed below is the Nazjatar region. Naga Queen Azshara, that lackey of N'Zoth, has always been a properly lowly watchdog. Expecting her to voluntarily raise her underwater city to the surface is utterly unrealistic. Only a brain-dead idiot would do that."
Standing at the