You can purchase Tormented Demonsoul Robes from Anwehu or Smith Hauthaa, vendors in Shattrath City and Isle of Quel’Danas.
Don’t forget to select ‘All’ in the drop-down menu when speaking to the vendors.

Two other versions of this dress (same color and design) are also available:

• Robes of Rhonin dropped by Archimonde in Hyjal Summit
• Robe of the Shadow Council dropped by Teron Gorefiend in Black Temple.