From Destinypedia, the Destiny wiki
The Parthian Shot is a Legendary Pulse Rifle introduced in Rise of Iron. It can be obtained from Roni 55-30 for Legendary Marks, the Vanguard from rank-up packages, or as a random reward from high level PVE activities.
The Parthian Shot is a member of the lowest-rate-of-fire, highest-impact family of pulse rifles. Although its sluggish fire rate makes close-quarters combat risky, each burst packs a punch and allows for a 2-burst kill against enemy Guardians in the Crucible at any range. Its inherently high range and low recoil make this a rather simple task, but stability is still essential for tight grouping. With the proper stability perks, the Parthian Shot becomes the fastest-killing legendary primary weapon in the game.