It is a piece of the dragon; a scale - enchanted and transformed. Whole, it could have granted access to the beast's lair. Shattered, it is useless.
It is possible to repair the item, but to do so would require the blood of a black dragon champion. There is one... He is known as General Drakkisath, ruler of the dragonspawn armies of Nefarian. Perhaps you have heard of him?
Return with his blood, <race>, and I shall enchant the fragment into something that will grant you access to Onyxia's lair.
Progress
The flow of time does not stop or slow, <race>. While my life is eternal, yours
is not. Waste not the time that you have...
Completion
This amulet will grant you the access that you seek. The lair of Onyxia can be
found in Dustwallow Marsh. As long as you hold the amulet, her wards cannot
prevent you from entering.
Pretty much as stated. Take a raid party, go to upper Blackrock Spire, and
emerge victorious.
The quest [60R] General Drakkisath's Demise also requires
defeating General Drakkisath. As Blackrock Spire is no longer on most
guilds' regular raid roster, you may want to perform both quests at the same
time.