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.