After you claim the [b]Legend Forged[/b] Triumph you are meant to get your free Exotic from the [b]Altar of Relativity[/b] vendor. If it is not showing up when you check, others have reported that they were able to receive it by checking the vendor in an instance hosted by a fireteam leader who has also claimed the Triumph. If neither works you should at the very least be able to get it to drop by performing well enough in the Sieve Activity to spawn the big reward chests. (΄◉◞౪◟◉｀)