Speak with Ak'Zeloth at Far Watch Post in the Northern Barrens.
Since the last great war when the Burning Legion was defeated, I have searched for sources of demonic corruption in orc society. The collar you brought me confirms my fears.
It belongs to the Burning Blade.
Your role in this tale is nearly complete, <name>. Go to Far Watch Post, on the border of the Barrens to the west, and speak with my assistant, Ak'Zeloth. Pass on to him this information.