Question: destiny 2 how to unlock dungeon?
Answer: Speak with Eris to get the Deepening Wake quest item. Travel to Sorrow's Harbor and participate in the new activity, Altars of Sorrow. ... 
You must clear at least a Tier 3 wave in order to complete The Deepening Wake. ... 
Once the Deepening Wake quest is completed, speak with Eris to unlock the Pit of Heresy dungeon.