The Blind Well
The Blind Well is a horde mode for the Forsaken expansion which is similar to Escalation Protocol and Court of Oryx. It offers several tiers of difficulty in which players fight through waves of enemies for a chest which rewards Dark Fragments, Legendary Gear and most importantly a Seed of Light.
Seeds of Light are the only way to unlock a new subclass and are a rare drop from this activity.
Petra Venj also offers several bounties which require Blind Well completions.