From Destinypedia, the Destiny wiki
The Origin Story is a Common and Legendary Auto rifle in Destiny 2.
It can be obtained only from completing the Strikes milestone (given by Zavala when he returns to The Farm). Once you complete this milestone, you have a choice of three weapons, the Nightshade, the Nameless Midnight and the Origin Story. If you don't choose the Origin Story, it can also be obtained from Vanguard Tactician Engrams.