"A dark mysterious emblem found in Dol Guldur. This can be exchanged in Emyn Lum."
Quoted from: Aruen
This is a barter item which trades in for a helm that always has +30 Radiance on it. To equip the helm , you must be level 65.
This item drops the "Barad Guldur" raid instance (need boss name). In my opinion, you need to be level 65 and have 115+ Radiance to venture into Barad Guldur.