Speak with Ak'Zeloth in the 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, a cult that rallies around an item of demonic power. It is called the Demon Seed, and it resides in the Barrens atop Dreadmist Peak. It must be destroyed!
Go to Far Watch Post, on the border of the Barrens to the west, and speak with my assistant, Ak'Zeloth. He will direct you further.