The Drakefire Amulet was once the key to Onyxia's Lair.
The attunement requirement, as well as the entire quest chain, were removed in patch 3.0.2. The questline, or at least part of it, seems to have been reinstated. There is still no attunement requirement.
See Onyxia's Lair Attunement for details on the old attunement process.
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.
This item is a quest reward from the following quests:
Be aware that if you partake of a Faction change 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.
Players simultaneously utilizing the Scroll of Resurrection and faction-changing back to their original faction may receive their amulets back in their welcome mail. The faction change was completed prior to logging in.