Netherspace



Netherspace is a region that is part of - but somehow exists beyond - the walls of Karazhan, and is the location of the tower's new master, Prince Malchezaar. The top of the tower can be seen from the other side of this "room", but upon going out onto the Master's Terrace, Netherspace is not visible. In Netherspace, there are several easter eggs if you look into it like a Skeletal Gryphon and a sleeping Nightbane.

Speculation
Netherspace may be shadowy mirror image of Karazhan where Sargeras lurked, mentioned in The Last Guardian.

It may be the "world other than our own" than an exact copy of Karazhan exist in mentioned by Archmage Alturus. Players may have entered in one world, and then exited the tower in a way that took them to the other.