The Human Heritage Questline was added with Patch 10.0.7, heavily involving the past confrontation between the Stormwind forces and Onyxia, then disguised as Lady Katrana Prestor.
Characters who completed the original questline in Vanilla WoW that led to the creation of the Drakefire Amulet (which served as an attunement to the Onyxia’s Lair raid back then) are able to experience a slightly different version of the questline, with NPCs acknowledging your participation in the events!
