https://www.gamespot.com/articles/destiny-2-weekly-reset-malfeasance-is-out-easy-see/1100-6462028/

When playing Gambit, a special Primeval can now spawn in that drops an item called the Seething Heart. This leads to a series of quest steps (some of them quite difficult) that ultimately results in the acquisition of the Exotic hand cannon Malfeasance.

Forsaken's new subclass branches are unlocked by getting your hands on rare items through Seeds of Light. The first of these is guaranteed early in the DLC's story, while the second and third were previously reserved for higher-end activities. The third, in particular, could be very difficult to obtain for many, as it was locked behind the Raid. However, a new development involving Petra's weekly bounty means you can easily get a Seed of Light--and your third subclass branch.