Hey there, when you played 'The Invitation' on Legendary, it also incorrectly marked 'Commencement' off on the Legendary triumph. That likely caused you to skip that mission when you were doing your Legendary replays, as your only completion of that mission is on Normal. The above bug let you finish the triumph but your Warlock does not qualify for the exotic unless all missions are finished on Legendary. Replay Commencement, on Legendary, and then check the Altar of Relativity.