Tenarus

Tenarus is a small town and fortress located deep within the Plaguewood. The small town was attached to a fortress built centuries before the present day. The fortress, taking the same name of the town, was built atop a hill where a cave was located. This fortress was made of tall and thick obisidian walls and was one of the few places in the Eastern Plaguelands where a holdout was established. The Scourge eventually took it over due to starvation and disease being spread into the fortress.

Following the fall of the Lich King, the fortress went abandoned for some time, before being reoccupied by the Order of Dusk in late 627 K.C.

Cave of Tenarus
The cave of Tenarus, in ancient Lordaeronian mythology in the ages before the Light took hold, was said to be the location from which one could enter the underworld, and as such, thousands of bodies were buried under there up to the present day.

Some Argent scholars argue that this cave was only recently uncovered and exploited by the Scourge following their second occupation of the Fortress of Tenarus in late 627 K.C. This cave, it's though, gave the Scourge access to thousands of corpses.