https://www.gamespot.com/articles/how-to-get-malfeasance-destiny-2s-exotic-hand-cann/1100-6462059/

A hidden quest in Destiny 2 has a special reward for those who go through multiple steps: the powerful Exotic hand cannon Malfeasance. You'll need to do a bit of footwork for it, but the stats and perks make it well worth your time.

As detailed by user ADT7 on Reddit (and confirmed by GameSpot), the Seething Heart quest pops up organically in some Gambit matches. An Ascendant Primeval Servitor will sometimes appear, and defeating it opens up the Exotic quest. From there, the steps are as follows: