The prison of Teron Gorefiend.
Tailoring
This altar is required for making the Tailoring item, 
[Shadowcloth]. It is located in the central southern edge of Shadowmoon Valley. You can see it on the map as a small altar icon directly south of the Scryers' Sanctum of the Stars base where the green falls go off the edge.
Coordinates (Shadowmoon Valley): 58.71