Pit of Heresy

Pit of Heresy is the second Dungeon in Destiny 2 and takes place on the Moon. It was released on October 29, 2019, inside the Shadowkeep Expansion as a part of 2019's Festival of the Lost. Pit of Heresy is also where the Exotic Machine gun, Xenophage can be acquired through the quest Emergence.

Guide
Before beginning the Pit of Heresy, the Shadowkeep DLC is required in order to start the beginning quest in order to unlock the dungeon. To unlock Pit of Heresy, the quest The Deepening Wake needs to be completed.

The Deepening Wake
''Travel to Sorrow's Harbor on the Moon and investigate the Hive rituals happening there. ''


 * Open a Tier III+ chest at the Altars of Sorrow.

"Luna's fate darkens with every step the Hive take. I can smell their desperation. Crota's daughters... they intend to turn it to destruction." -Eris Morn
 * Defeat Nightmares at the Altars of Sorrow. (20)

The quest requires you to defeat Nightmares while participating in the public event Altars of Sorrow. Altars of Sorrows is not too dissimilar to the public event, Escalation Protocol in the sense of defeating waves of enemies with objectives to complete like defeating bosses. After that has been completed, return to Eris Morn to unlock the dungeon.
 * After completing three levels of Altars of Sorrow, chests will appear which contain loot. This is what the requirement is referring to.
 * Nightmares can be distinguished from regular enemies by having an orange aura around them. They also usually have large amounts of health and dealing much more damage than regular enemies.