You must retrieve the Blood of the Black Dragon Champion from General Drakkisath. Drakkisath can be found in his throne room behind the Halls of Ascension in Blackrock Spire.
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.