Drakefire Amulet

The Drakefire Amulet was once the key to Onyxia's Lair, until the attunement requirement was removed in patch 3.0.2.

See Onyxia's Lair Attunement for details on the old attunement process.

Background
Drakefire Amulet, is a ritual object created with the help of General Drakkisath, the leader of the dragonspawn in Blackrock Spire. The amulet requires some of Drakkisath's blood to function. It was the only way to get past the powerful wards that prevent anyone from entering Onyxia's Lair.

Source
This item is a quest reward from the following quests:

Item deletion
To restore the key simply talk to Haleh (Alliance) or Rokaro (Horde) and you will receive a new key. HOWEVER, if you partake of a Faction change that the item will be removed from your bags completely due to the removal of Faction-specific quests and their rewards. The option of talking to your side's NPC to restore it is not available. It is unknown if changing your faction back to the original side restores the amulet.